I've owned Dells for decades, have bought lots and lots of them, and like them a lot. They are very inexpensive for what you get, but yes, upgradability is not a strong suit because the motherboards and cases are non-standard formats.
I've put stock power supplies in the past into Dells -- it depends on the Dell model. Look at the technical specs for your Dell (download it from the Dell site) and see if a standard power supply connector will work on it. (Make sure each wire in the connector is listed as the same voltage as what the connector in your Dell model needs.)
As for a graphics card, most these days cards in the $70-$90 range will be plenty fast for Aces High. The problem is that you have an AGP slot, which is an outdated slot. Most PC's now use PCIexpress (PCIe) slots (not that same as PCI).
You can look on newegg.com (excellent prices in general) to see what they have in the $70-$90 range for AGP and look at the chipset's ratings on Tom's Hardware VGA charts (
http://www.tomshardware.com/charts/graphics-cards,1.html ) to see how it does. Even older Nvidia 6800GS or 6800GT cards (and things with performance in that range) are plenty fast for Aces High, in my opinion.
