Cards like the Nvidia 8800GS/9600GSO or Radeon 4670/3850 will fit in that budget. A 9800GT will also scrape in at just over that, 8800GTS 320MB is also available. That's if you want a graphics upgrade, as that is the weakest part of your system now but if you're not much of a gamer, it's not really worth it.
Alternatively, a faster Athlon X2, the 6000+, could be a good upgrade, or a Phenom X3 CPU.
I agree that those are the two areas that would benefit from an upgrade.
The Vista experience score is out of date, it was supposed to get regular updates to take into account newer components but it never has, to my knowledge, so a lot of things will score highly because they were good, compared to other hardware when Vista was released, if that makes sense.