your trouble is that you have no options under your current system. You cannot get what you want because what you want is not made
My advice is to go over to newegg.com and get the cheapest Raidmax case you can find. Won't cost you more than $30. If your dell's power supply is a standard ATX, then you can get the case even cheaper by dropping the PSU from it.
Then go get a motherboard on pricewatch.com. Cheapest PIII board you can find that has AGP 4x or better slots. That also shouldn't cost more than $30. That's $60 to go from no options to being able to choose any video card you want.