I think the only thing you can do is upgrade the RAM to improve the video performance. The HD being at 4200RPM will still slow you down so it won't be much of a improvement but every small gain helps.
I don't think you can add on video memory without some major modding. But video memory does help much (even if you are able to find a way to add it on). For example the 128MB and 256MB 9800PRO have basically no difference in performance wise (same with other cards with same GPU). It really depend more on your GPU. The 9700 is fairly OK and I think is the best you can do on a lab top. The power supply in the labtops wont have the power to run the 9800 or upper as far as I know. Labtop is made to be compacted and have very limitted resources. Small powersupply, hard drive runing slow, low voltage CPUs, etc.
If you want good performance video, you'll have to get a desktop (or at lease a micro computer like the Shuttle).