BTW. That is actually a Uniwill 258KAO barebone laptop that is configured by ABS Computer. I don't think you will be able to upgrade this to a Mobility 9800 or x800 series GPU since the cooling requirements will be different. I also didn't see any upgrade options at the ABS site as well.
I would look at other options like laptops with the mobile x700, x800, or Go 6600 GPUs. A few years ago some laptop manufacturers ( Dell, Sager, Alienware ) flirted with upgradable GPUs but that has disappeared. I guess it wasn't that popular or too much trouble. I have also seen the trend where only 17" laptops have a Go6800/7800 or Mx800, 15" have a x700 or Go6600, 14" have x300 or similar, and the 12" have integrated graphics. Unfortuantely, if you want a gaming laptop, it is going to be a HEAVY and HUGE one.
I have a Dell Inspiron XPS gen 1 ( 3.4 Ghz P4 ). It origionally came with a Mobility 9700. I upgraded it to a Mobility 9800 and got roughly a 40% increase in playability. Even the M9800 is barely adequate for smooth gaming. It would help to get a laptop with the lowest acceptable ( for you ) LCD resolution and keep the eye-candy to a minimum.
If I remember correctly, my 3D Mark03 scores on my XPS were 4200 for the M9700 and 6500 for the M9800. Having the AMD will get you a few more points.
Here is a mobile GPU comparison guide which should help you determine what one you want to go with.
http://www.rojakpot.com/default.aspx?location=3&var1=98&var2=0