I was thinking to get a AMD Radeon HD 4870

I have a pci-e 16x slot :[ but to make matters worse, it's locked at 8x.(impossible to chnage it without changing the motherboard)
How much lowered performance will I recieve if I use the 4870? What about the Nvidia GT2XX cards?

I remember in your other thread you said the PC was a Dell, right? Have you tried calling them and telling them about this? The 650i runs at 16x, hell any motherboard does that supports pcie that isnt 3 years old running on a budget microatx chipset. But anyways...

The 4870 is a badass card, point blank. You cant go wrong with the purchase at $300. However even at $350-$400 the GTX260 is a ripoff as the 4870 is stronger, and the GTX280 at $500-$700 is just plain silly. There are reasons Nvidia stock droped 22% the other day, and they had to pull out "emergency price cuts" on flagship models.

None really, the only difference youll see is in 3dmark to be honest. And in software that requires mass vram like microsoft flight sim.

alittle offtopic, but when you sli, it goes to 8x even if your slot is 16x or something?

Some motherboards don't support two full x16 lanes simultaneously. They just don't have the required bandwidth. So instead, they provide two slots but only the bandwidth of one slot to power the two, so instead of one x16, you get one x16 over two slots, or two x8 slots.

nvidia cards for SLI (and only some of those)
ATI cards for crossfire (and only some of those)

one more question! sorry ugys you're just way too helpful haha:
Out of the GT2XX and 9000 series cards, which ones can you SLI and how much of a power supply would you need?
what about if i pick up another 8800gt? does it matter what manufactor I get it from? Will I have to put my current 8800gt back at stock settings? Anything else I have to pick up to put in the computer with the second 8800gt?