Interesting. My family has an older dell box downstairs which I used for a long time before I got my MBP...after using OS X for a few months I really dislike the windows interface, but totally understand why you need the dell. If I had the money to spend between a high end gaming PC and a Mac Pro, I would choose the PC because it is simply aimed towards gaming and I don't use enough pro-apps to utelize the Pro to its full extent.
I think when you get the better card in there and maybe up the RAM (how many slot does it have?) it should feel a good bit quicker. If I can play HL2 at high graphics, 2x AA, HDR, and 1400x900 resolution on a ATi x1600, then you should be able to do even better with a better card than the 7300.
It is true it is not really in the same catagory as the mini, but neither is it in the same catagory as the iMac or Mac Pro...it is low-end tower, something that apple does not make. However, the price point is closest to the mini, and seeing this is an Mac forum, I think the comparison is justified.
Good review, and I hope to hear how it performs with the new card.