Upgrading an existing computer is often the cheapest option for gaining more speed and reliability. Be aware though that not all computers can be upgraded sufficiently to cope with the latest software and this should be carefully weighed against having a machine purpose built or buying a new one from your local retailer. The latter, given the retailer's buying power, can often result in some great deals in terms of extras (like printers, scanners, etc.) giving you intially far more "bang" for your bucks.
However, the downside to this option is the limited upgradeability ("future proofing" as we call it) retail machines can offer. Check out BUILDS above.
The typical items that are best upgraded are the CPU, the RAM, and moving to a larger Hardrive. All these will be dependent on the Motherboard capabilities. Upgrading to a new Motherboard will be determined by the formfactor (industry standard) of your machine. Early machines were in the AT formfactor but nowadays the ATX formfactor is where technology has headed.
If you require any advice or help with these decisions, contact me.