I built a high performance system out of my Dell 4300, which suffered a motherboard failure. I kept all the drives, keyboard, sound card, the new ATI 9600XT, modem, speakers (with amp) and the monitor.
I bought a new case, motherboard, CPU, power supply and added another hard drive.
For considerably less than $500, I have a terrific machine that will not require serious upgrading for at least two years (see specs below)
The only limiting factor is the 85 Hz monitor, but that is still adequate for running everything, including AH2. With Vsync on, I'm pulling 80-85 fps with the graphics turned up to max, virtually the same as with AH1.
So, you guys with older machines can still upgrade without buying a whole system. If you are confident that you can do the work yourself, you can save considerable money. Typically, all you really need is a new case, motherboard, CPU and maybe new video card.
My regards,
Widewing