hi i have a 9800gt and i was wondering whether i should get another 9800gt for sli (i have an sli-ready mobo, but it would only rum x8 x8 sli) or just get an hd4870/gtx260 and use my 9800gt for physx. ill be playing at 1920x1200
my pc specs:
asus p5n-e sli
4gb 800mhz ddr2 ram
chieftec 650w power supply
dual 9800s' will be faster the only cevat here is your choice of settings: If you like AA then the better option would be the GTX260/9800 PhysX combination, the GTX260 will handle AA better at that high resolution and is faster than the 9800 giving a nice double-edged upgrade. If you keep AA low or off then dual 9800s is the better option, but read the SLI FAQ at the top of the front page, SLI has some quirks you should understand before making a decision.
Either way the PCI-E bandwidth is a non-issue, neither the GTX260 nor 9800GT are fast enough to use a x8 slot to its maximum capacity and PhysX cards only use/used PCI-E x1 slots.
Honestly this is one of the few times I would actually recommend buying another 9800gt. Like coozie said, the 8x won't actually create any bottleneck problems. It's a pretty tough decision. Another 9800gt wouldn't be a bad choice though. Those specs are good enough to last a few more years, so I would definitely stick with an nvidia card for potential SLI. 4870 would be a waste.
Not a lot between SLi GTs and a GTX260, and an extra GT is probably the cheapest option. But as your res is high I'd look to something with more than 512Mb of texture memory (SLi 512Mb GTs is still only 512Mb - Not 1024Mb), so with the future in mind I think the GTX260 216 or an ATi 4870/58x0 would be better. (Unless you have a 1gb 9800GT already).