My spec P4 machine was bought brand new in 2005, but then it had a 9800 All-In-Wonder Pro and a different motherboard. At two different times, the motherboard blew the caps and the card blew out too. So now my machine is as it currently stands and is still kinda awesome for a P4 machine. It plays most of the latest games very well still, but some like the new Need For Speed Hot Pursuit, it only gets 10 fps.
I added a SATAII card and a WDCaviar Black 1 TB along the way as well.
I have another machine running Windows 7 flawlessly from 2001 and it's a non-hyperthreading P4 with 768 MB RDRAM and an NVIDIA 6200A-Le. It is perfect for interent, which is all it is really used for. It can play games well of the time and slightly newer.
Your machine is still rather excellent by any standards. No need to upgrade, IMHO.

Maybe a better video card if you game at all and want to keep up with the times, but that 8800GTS is rather good still too, heh heh.