This question has more answers than the meaning of life, so all answers are going to be opinions. My personal opinion is: Never upgrade the Motherboard/Processor combo unless you can double the speed. Someone will gig me on this, for speed is only one component of the big picture on your system, and they are right. This is only a rule of thumb. Throughput, memory type, bus architecture, features, personal use (Office, multimedia, or games) etc., etc., should all be taken into account. If an Office style user, why bother? That 486DX33 is just fine! (Just kidding) If a multimedia user, which option: Upgrade or new system is most efficient? That would depend on Software, and the capabilities of software version. Are you using the latest edition of Adobe Premiere that recognizes Hyper-threading? Then new system! The difference is astounding!! If an Avid hardcore Gamer doing lan parties: You should live next door to Comp Central, with a personal account. Back to personal opinion: If you can double the speed, try to refrain from store bought systems that use proprietary components. Certain systems on the market such as HP and the like have an army of Engineers that basically build their systems from the Power Supply up. The concern you run into, is compatibility for video and memory upgrades in the future. Stick with Builders that use excellent quality off the shelf components such as AlienWare, etc. This makes future upgrades easy and won't require purchasing memory from only one vendor requiring a second mortgage. If you don't have AGP graphics, and you are not a strict office style user, a new system is in order. If you have AGP and it isn't 8X, and only 4X then you are fine, and should do graphic card and memory updates. (Max memory speed of system?) Check with your system's manufacturer and find out the maximum speed processor your MB can handle, and see if you can max the processor out. AGP8X, serial ATA, and all those beasts are fixes to non existant bottlenecks. No game on the market can max a 4X AGP bus (Doom3 will, but when is it coming out?) No ATA hard-drive can max out a 100MB/s bus, so why upgrade to 133 or SATA150?

Redpen's answer is good, but here's a simple way to look at it: right now, if you went to the local computer store (or online), could you buy a faster processor for your current motherboard? One that isn't used? If the answer is no, and you are noticing slowness and lag in your daily computing, you need a new motherboard, which means you need to buy a whole new computer anyway. If you want to, building a computer by yourself isn't all that hard and there are excellent guides available online. On the other hand, if you do so, tech support is hard to come by, it takes some time, and if you break an expensive piece of equipment you're on your own. If the answer is yes, then I would recommend upgrading individual components to acheive the results you want. Be aware that what you should upgrade depends on the problem. For general lag and slowness, try getting either more RAM or faster RAM. If you already have 512 megabytes of RAM and it is DDR-333 or higher (also known as PC3000), then your RAM is probably fine. After RAM, try upgrading your CPU. I like AMD, as they are more cost-effective, but it depends on your motherboard. See if you can get a CPU with a faster front side bus (FSB) -- that will speed up the rate that your processor "talks" to the RAM. Again, the FSB is also dependent on the motherboard. If you have plenty of good RAM and a decent CPU and you're still having problems, try defragmenting your hard drives and running software like Ad-Aware or Spy-Bot. Do not upgrade your video card for general slowness, unless your video card is very old or is integrated into the motherboard. Video cards will generally only help with 3-D games.

I'm being very generic here, but this should help too. It really depends on how old your computer is. Lately, I've been finding that computer architecture is changing quite rapidly. So a new part you buy today may require (or have as an option) a new interface that you don't have. A new part may offer speed or capability that your computer cannot take advantage of, either because of an older interface, older BIOS, older operating system, etc. I'm also noticing it's less likely that you'd be able (or want) to salvage parts from an old system and bring them along into a new computer for the same reasons. So, the blanket answer is 'yes'.

It is always much more cost effective to build your own system. Granted, alot of people are clueless as to what a motherboard really is, there are always people there to help you. Check out your local computer club (Almost every city has one or 20), they love to help. Not only that, but the feeling you get when you successfully build your first computer is great. Everyone has a different need for their computers. Thats another reason to upgrade/build your own. An off the shelf computer may not give you everything you need, and at the same time, give you too much of what you dont need. Here are some advantages and disadvantages to both: Buying a new computer Advantages: Easy setup, preprogrammed, no need for any technical knowledge. Disadvantage: Higher price, little control over whats inside, Gotta take it in for repair (Or mail sometimes) which can sometimes take a long time. Building a new system/Upgrade Advantage: Learn more about your computer, costs less, great control over whats inside, can fix yourself if something breaks. Disadvantage: Takes a little time, Gotta put software on yourself. There ya go :) Hope it helps!