HP? Do you mean an HP computer? If so the first thing I would check is what rating the power supply is as it may need to be replaced. You mention SLI do you have a SLI motherboard? IE 2 PCI Express X16 graphics card slots?
I got myself a 8800gts 640 and its an amazing step up from my old 7800GT. All the core2duo are great processor with whopping OC potential. With that rig ul have a really good all round performance. I dont think SLI hads really taken off with very few games being developed to take full use of the 2 gpu.
I think the SLI support is still a bit ropey with Vista yet and to be honest you'd be better sticking to one card at the moment. PCAs best buy is the BFG 8800 GTS 640mb and supports directX 10 costs £292 but I'm sure if you shop arround you could beat that. Yep £258 click here or the one with less memory click here just over the £190 mark.
Definitely go widescreen, its becoming more and more supported and makes games alot more engrossing, what sort of money are you looking to spend on the monitor? Id recomend the 640mb version as it will be alot more future proof than the 320mb. From reading around games in the pipeline, DX10 is going to be able to produce some astounding textures and effects which will suck up gpu memory.
As citadel said a good power supply and cooling is a MUST for the 8800 its a massive card and runns very hot.