Right now I have a Dell Demension 8300
3.00 gigahertz Intel Pentium 4
512MB RAM PCI 3200
NVIDIA GeForce FX 5200
You have the start of a great computer... for $1000 you can make is amazing.
Take $240 and get 4 512MB RAM sticks. (I just paid $120 for 1GB 2x512)
Your motherboard probably doesnt have PCIe so just get a kick ass AGP card for around $300.
Your processor is good enough in my mind, now if you had PCIe and were going with a kick ass card I would recommed something bigger in the processor.
The RAM and video card are plug and play, so if you can get into your case you should be fine. Your power supply might need a boost so if you can find out it's specs it may help too.