USB 2.0 isn't fast enough to feed a graphics card with the bandwidth it needs. There are other external graphics card docks that use ExpressCard  interface but from my quick search they seem to be very expensive for what you get.... (>$200 for the external enclosure alone, not including the graphics card which might be another $100) and are difficult to obtain.

IMHO get an internal graphics card but a quick search reveals that:
1. The power supply in your computer is extremely weak at 220 watts of unknown quality.
2. The case has a small form factor and can only fit half-height videocards.

Which means your choice for graphics cards is extremely limited or non-existent - which is the kind of thing that happens when you buy a HP, Dell, Acer, Gateway (etc) especially in this form factor. Fastest low profile graphics cards are as far as I know, low profile versions of the nvidia GT 440 and AMD Radeon 6570... but I really have no idea if they will run properly on that power supply and while both are a big step up over what you have now, neither are very fast.

You could ask a computer store to test it and put one of those graphics cards in (or do it yourself) but that might incur extra cost that may not be worth it especially if it doesn't work.


If it were my computer, however, I would do one of three things.
1. Keep the computer as it is.
2. Sell it and buy a new computer not from Dell, HP, Gateway or other OEM. Instead buy from a proper computer store with parts chosen by people on this or other forums.
3. Buy a cheap but quality new: motherboard; case; powersupply; and graphics card. Throw your existing memory, CPU and hard disk into the new parts, and throw away or sell the unused parts.

I like option two the most.