if you got a mobo that supports SLI then you can connect two identical GPU's together for 2x performance, but the performance is dependant on drivers and the software you're running.
if it doesn't support SLI its still faster than 1 card however its not that fast.
current drivers don't exploit the PCIe cards potential to 100% yet don't say SLI which is an even newer technology for PCIe.
so it basically means that if you buy a GeForce 6800 Ultra in Q1 2005 for RM1000 its going to be the fastest. by q4 2005 or q1 2006 nvidia would already lunch their next generation GPU maybe called GeForce 7800 Ultra at RM1000
so you could spend RM1000 to get the next generation performance or you could buy another GeForce 6800 Ultra (maybe at RM500) and run both your cards in SLI mode to have the same performance threshold as the next generation card.
saved rm500 but same performance. performance as in processing power, fill rate, etc. however the next generation card maybe have higher spec.
eg.
6800 supports DirectX9, Shader 3.0, OpenEXR 1.0
7800 supports DirectX10, SHader 4.0, OpenEXR 2.0
thats just for example, hehe, there is no 7800 out, yet :P