You can upgrade the RAM to 4 Gig of DDR2 PC2-5300 (4x1Gig) and depending on your power supply (I think you have a 300W) you can get a HD5670 or HD5750.
As for the CPU stay with the 805, If you upgrade I think the max is a Pentium D 900 Series, But it would not be worth the cost, IF you want to run a newer CPU you need to replace the motherboard, Dell uses BTX cases and motherboards for most of the Pentium D computers.So you would have to get a new Case, Motherboard, PSU.
I have a Dell E510, Iwould like to know if you could tell me what's the best part's I can install, service tag # 9VCVQB1 , processor, ram, video card, etc.
Dell Dimension E510
Windows XP Media Center Edition 2005; 3GHz Intel Pentium 4 531; Intel 945G chipset; 512MB DDR2 SDRAM 400MHz; ATI Radeon X300 SE; Maxtor 6L160M0 160GB 7,200rpm Serial ATA