Depending on the type and amount of ram on the 8800GT, a 9800GTX may not be that much faster. Also, your cpu may not be capable of driving a high end card to it's capacity.
For a significant jump in performance, you should be looking at a video card like the GTX260-216 or the 4870-1gb.
If your motherboard is sli capable, a second 8800GT would do the job also.
To drive that card, you should be looking at a cpu clocked at 3.0 or higher.
ATI or Nvidia should make no difference as it relates to the cpu. Only if you are considering multiple vga cards does the chipset(ati/intel) make a difference.