Sis is a very weak, little performant GPU, its that simple.
On my last computer I had an onboard chipset, and I replaced it with a Sis graphics controller but found that the Intel chipset was way better.. ;D
However, yes indeed a graphics card will make a difference in perfomance and I definately saw a huge rise in frames and graphics perfomance on my new system.
If you want to have a look at price ranges:
Geforce 7800 GPU's (newest and most powerful), mine cost me about 450Euros.
Geforce 6800 still extremely good, you're probably looking at around 300Euros.
Geforce 6600 is more affordable, probably in the 150Euro range
Geforce 6200 and below (Fx5200 etc) are now pretty cheap, I would say you could definately find one for a 100Euros, but nowehere near as powerful as the bigger GPUs above.
Heres my chart again!
Relative 3D Graphics Perfomance
Geforce 7800 GPU's: 41X
Geforce 6800 GPU's: 24X
Geforce 6600 GPU's: 16X
Geforce 6200 GPU's: 5X
Of course you can also looks at the rough equivalents of Ati Radeons, in terms of performance I think the order is something like:
1)X850
2)X800
2)Radeon 9800
3)the lower numbers
I may be horribly wrong though because I know little about ATI GPU's.
However, its not only the GPU or Graphics card thats important. You need to look at everything, I bought a whole new system altogether with components that are performant and work well together. For example, my mobo is built specially for my GPU, the Geforce 7800GT. You may want to consider that, older mobos may not have any PCI-E slots anyways to fit a newer GPU.
Hope it helps :)
Alpha
