Will a Geforce 7900GS allow me to turn on many more options than the Geforce 7600GT? If not I'll stick with the 7600GT even if it is only 128bit.
The 7900GS is a better card, therefore more options are available to you as the card can handle more data thrown at it.
Is it absolutley proposterous to risk buying a 500W PSU below fifty pound if the customer reviews say it is ok?
"cheap" PSUs that claim to offer 500w often break very quickly and closer examination reveals that they often don't provide anywhere near the amount of power printed on the label.
Therefore, a higher quality PSU that provides only 450w is better than buying a cheap and cheerful 500w one. Me, I'm running a Socket 939 64-bit processor on a 350w PSU, but it was a more expensive (and thus quiet) one, therefore I'm happy to use it over a 450w one that sells for £20 and roars like a hovercraft, and may again break pretty quickly.
Should I bother with a sLi motherboard? I won't be using it immediately anyway. Will I be able to put my new graphics card to good use in a few months time when I buy a DX 10 card?
Not at this time IMO, as they're more expensive and unless you're really heavily into high-end gaming, not worth the money either.
Is it ok to put a DVD drive on the same IDE cable as my hard disk? I intend to use my old 7200rpm 8mb cache IDE drive to save a bit of money but I noticed all the new nForce 5 motherboards only have 1 ide connector.
You can do it but the hard drive will only run as fast as the DVD drive, which is often only half the speed of the hard drive. Therefore your 8mb cache won't make much of a difference.
New boards are now disappearing off down the SATA route - the drives are faster, performance is better, one doesn't need to worry about jumper settings (literally plug it in and go) and generally becoming the standard. One can now buy SATA CD units as well, so I reckon in less than two years time there will be no IDE ports on a motherboard at all.
Is it worth getting ram with speeds above PC4200?
Indeed. PC4200 = DDR2-533. The 3800 is capable of running with DDR2-667 or DDR2-800. The faster the speed of the memory, the quicker Windows (and everything else) works. There is a real world of difference between DDR2-533 and DDR2-667, especially if you see a system running with the 533 memory for a while and shove a stick of 667 in it. XP literally flies.