In answer to your question, i have used both a 7600 and an 8600, the 8600 is by far better IMO. performance wise, and it also supports DX10. and to answer your other question, if you plan on running single cards, the Radeon 3850 is by far the better choice, i upgraded from my 8600 recently and WOW. But for what you told me you do, you dont need Dual cards. a single 3850 or 3870 will suffice.

does windows XP support direct x10 graphic cards or do i have to invest in Vista?

No. Only vista currently. But its worth investing in a card that will support it for later, because eventually DX10 will either migrate to XP as well, or you will have to get vista. the horror of it =P

Ill defo get the 8600GT as a graphics card but reverting back to motherboards....

would you pick the Asus M2N-E SLI or the Gigabyte M57-SLI and why ? (the main factor to me is being future proof for as long as possible)

Just my 2 cents worth.. I love gigabyte boards.. They have never done me wrong. Although Asus has some problems.. I love the BIOS features gigabyte has to offer like Xpress Recovery Bios recovery and has a second piece of ROM on board just in case.

And I agree about ThermalTake cases.. I have one.. Mine is a little more pricey but keeps my pc nice and cool

Newegg.com - Thermaltake Armor Series VA8000BWS Black Aluminum / Steel ATX Full Tower Computer Case - Retail

merlin, what model is your motherboard do you have and would you recommend it?