I Have 2 Nvidia 8600 Gt 512mb cards.
I do not have a Sli ready Motherboard and am wondering if it is worth purchasing "nForce 750a SLI" which is around £80 for me in the uk which will enable me to Sli my two cards. Or would I be better of getting a better single GPU?
definatley not you would be better off with a single gpu the 8600s are pretty darn weak and imo not worth 80 to upgrade, especially with your lowly system. I would hold on to what you got and save for a new build, your ram and cpu are going to kill off most of your potential gaming performance, I would at least go for an am3 build and if you can afford it a Sandy Bridge build would be excellent for gaming, the i5-2500k is really an attractive buy right now, shoot even the 2400 is nice.
so to sum things up basically Im saying your computer is not worth spending money on upgrades and you should start with a clean build and go from there, hang on to your current rig and 8600 gt's at least so you have something until you can afford to build a new rig. If you are against building a new one, I wouldn't recommend getting any gpu higher than a gt 240 or an hd 5670, in fact I would probably recommend getting an hd 5670, and getting 2 more gigs of ram and try to run them at at least 667mhz or imo it will seriously bog things down at times.
My Ram is Corsair Dominator DDR2 1066Mhz which my motherboard wont run it due to it not supporting higher enough voltages. So would it be worth trying to get a Motherboard that will support the memory, needs 2.1/2.2 v and is Sli Capable? Then getting another 2 gb of ram When budget allows?
lol sli has nothing to do with ram and yeah youd benefit from being able to run 4 gigs at 667mhz or better but your system is weak and sli'ing 8600 gts will likely still be less powerful than a modern $80 gpu is, so in the end jsut get a low end single gpu such as a 5670 and fix your ram situation if you refuse to build a new rig. Sell the old ram ddr2 is still worth a decent amount on ebay, also ebay your two gpus,____ then get proper ram and buy a 5670 or better, a 5770 would be nice but might be out of your budget, that would be a much better option then spending 80-100 on a new motherboard to attempt 8600 gt sli which i guarantee you will not be happy with. I would not recommend sliing anything lower than an 8800 gt which is far superior to an 8600gt trust me its not worth it. two 8800 gts are similar in performance to a gtx 460 in dx10 I'd say just to give you an idea........... dont sli the 8600 gts, not worth it