I recently upgraded my machine. My usual rule is to upgrade if it looks like I can double my performance with the latest technology. My old system looked like this:
1.2GHz AMD Athlon
512MB of NON-DDR SDRAM
Asus A7V333 Motherboard
30GB IBM Deskstar 7200 RPM drives (2)
Nvidia Geforce 3 Ti 500 Video
Soundblaster Live!
So the new system is as follows:
AMD Athlon XP 2100+
512MB of 400MHz DDR SDRAM
Asus A7N8X Nvidia Nforce2 Motherboard
80GB Maxtor 7200 RPM Drive
ATI Radeon 9700 Pro Video
Soundblaster Audigy 2
Performance in all games got either double or more measured by frames per second. The Athlon XP 2100+ is not the fastest chip on the market, but I decided to save money so I could upgrade to one of the faster chips when they drop in price later this year. Given that I am very pleased with the performance of the system. I had it put together for me by a local merchant who does a great job, and beats the price of most web sites. I enjoy knowing every component in my system, and knowing I can upgrade anything with off the shelf parts. I have several friends who have bought Gateway or Dell PC's and I am horrified with the problems they have doing even simple upgrades. Take my advice, if you want the best, and you want to be able to tweak it yourself, build your own, don't by the big name stuff.