It would make a fine addition to those system specs of yours. X600 to X1800XT; what an upgrade!
The only thing is my crappo 300w power supply. I've been having problems with it shutting down at random. First, I got a new CPU cooler, and reapplied the thermal paste. Then I got a new case cause the old HP case was too small and had terrible airflow. Now I'm giving it to a geek that I know who has other parts to swap and test.
I'm hoping that it's the CPU or motherboard, so then I can:
A) Get a new Intel motherboard, and try to OC to 4GHz+
B) Get a new AMD motherboard and an Opteron 144 or A64 3700+ Sandy and overclock that. And then get a new power supply. And another gig of RAM. And a new video card. And a floppy drive/media card reader. And then I'll basically have a new computer. I'll keep my hard drive and optical drives.