For that price, you could get a Nvidia 9800GT. Just falling short of the maximum amount:
There is also the ATI 4830 which again just falls short of your maximum:
I'm really not sure how powerful those cards are, but thats how far you can probably get, at the price point you mentioned.
LambOfGod beat me, but yeah, if you can afford $20 more, the 4850 will smoke most games you try. I didn't even hear of the 4830 before I just checked...
I believe the cards are single slot though which is a good thing for you
I do have to mention though, that your power supply of only 300 watts could be an issue, and it'd probably be worth seeing the recommended wattage for each card...
It looks like the minimum PSU wattage for the 9800GT, and probably any other cards of that power is 450W. The 9800GT can drag 225 watts from your PSU at full load, so all in all, I think an upgrade of your PSU is neccessary first
Or, you'll have to get a much less powerful card, which won't even risk getting close to the maximum or minimum amount you specified for the upgrade