Here's a suggested path to upgrade:Asus P4PE2-X
$54.50Kingmax PC2700 512MB
$39.99Panaflo 92mm FBA09A12H
That leaves you $265.76 for a graphics card, which can get you something pretty good.
If you dont mind, you can get the XFX 6800 non ultra
which is a very, very decent card, but will put you overbudget by around $35, or you can grab a Sapphire 9800pro
for $209.50 which is, in it's own right a very, very capable card, not as good as the 6800 non ultra by almost a 1/3 but ALOT better than the 9600XT u wanted to get, but that also leaves u with $56.26 spare, which can get you:Arctic Cooling VGA Silencer rev.3
$21.99Arctic Silver Ceramique
This is thermal paste for the interface between the heatsink and the GPU and CPU. For those who are whining for me to get AS5, i suggested ceramique because he's not an expert on this stuff, Ceramique is nonconductive so if he does make a mistake and "short" something with it, nothing will happen and secondly, it's cheaper than AS5, plus, the difference in performance is very, very close, almost negligable.
If you want to cool the ram on your 9800pro, you can also get:OCZ Copper BGA Ramsinks
Which is pretty much the best ramsinks in the industry today. Firstly, these add a bit of colour to your card as well as cooling the ram so you can get better clocks with it.
To stick them on, you can use a drop of superglue on 2 corners of the chip, then covering the rest of it with AS Ceramique, which makes it pretty much unremovable, or you can get:Arctic Silver Alumina Adhesive
Mix the two tubes with AS Ceramique in a 1/2 mix, so one quater of the mixture is the AS Adhesive tube 1, second quater is the AS Adhesive tube 2 and the remaining half is AS Ceramique. That bonds the ramsink relatively tight enough as well as being removable. You can pop it in a ziplock bag just to be sure, pop it in the freezer for about 15 minutes, take it out and rip the ramsinks off and the ram wont come with it.
Even after all that tuning to the 9800pro, you will still have $10.30 left over. The result will be a still rather quiet PC, but very, very overclockable. I would estimate the CPU to clock to about 2.6GHz to 2.9GHz, depending on how good your chip is, but dont blame me if it goes under or over those estimates as no piece of sillicone was created equal, every chip is different.
As for the card, 9800pros tend to clock very, very well with a new aftermarket heatsink and ramsinks on it.
I would think your machine after that will be able to play Doom3 at medium quality, 1280 x 1024 with half of the eye candy turned on as opposed to your original suggestion of a 9600XT which might be able to do medium to low at 1024 x 768 with most of the eye candy turned off.
In the end, when you want to upgrade your CPU, you would only be throwing away the CPU, Motherboard and RAM. The CPU u originally had and you thought u would throw that away, and the Mobo and RAM, you would only be throwing away $88.50 worth of equiptment. Everything else is reusable in your new setup.