That P4 3.2 will likely be quite a bit faster with the 5600 than the 1.2, considering it's close to 275% faster, has SSE2 and will be running dual channel ram so I wouldn't give up on that card entirely.
The 5600 should OC pretty well for even that much more gain and as slow as it may seem it will blow the 900 GPU out of the water since it has both hardware pixel AND vertex shaders plus it supports shader model 3.0 as opposed to just shader model 2.0 like the IGP in the 915 chipset.
Myself I've had 2 different cards in this machine since I built it back in 2003, the first was a FX5900 Ultra and the second was the current card which is my trusty 6800nu with all the pipes unlocked.
Since my chipset is AGP only there are no modern cards that will work with it.
Phenom II 940 Black Edition quad core @ 3.5Ghz
2x1 gig OCZ PC26400 Platinum, 2x1gig GSkill PC26400
Some other stuff,
Even more stuff,
If the automobile had followed the same development cycle as the computer, a Rolls-Royce would today cost $100, get a million miles per gallon, and explode once a year, killing everyone inside. --Robert X. Cringely, InfoWorld magazine