The 2.8 Celeron would be a nice performance boost, and not to expencive at $48. Yes it is a Celeron, a p4 would be better, but also more expencive ofcourse.
What would you about want to spend total on the new proc and video card?
This would be one of the best cards you could use:
XFX PVT70KUAD7 Geforce 7800GS 256MB 256-bit GDDR3 AGP 4X/8X Video Card - Retail
http://www.newegg.com/Product/Produc...82E16814150131
Thats $300..
Or half that price will get you something like this:
eVGA 128-A8-N374-TX Geforce 6800XT 128MB 256-bit DDR AGP 4X/8X Video Card - Retail
http://www.newegg.com/Product/Produc...82E16814130268
$150
And again about half of that:
JetWay 96XT-AD-256C Radeon 9600XT 256MB 128-bit DDR AGP 4X/8X Video Card - Retail
http://www.newegg.com/Product/Produc...82E16814153012
$80
Things really depend on youre budget, without knowing what you want to spend on it would be guess work I'm afraid.