Gaming performance will obviously be higher on the more expensive Shuttle SD37P2, because of the inclusion of the native 6-pin PCI-Express connector allowing for a much more powerful graphics card to be installed. On the other hand the AOpen and SS21T systems are limited to just 75W card through the PCI-Express x16 slot, so you're limited to a 7600GT (which we used), X1600 Pro, 8600 GT or HD 2600 something when it arrives.
This will seriously limit your gaming experience to a 17" or 19" TFT and 1280x1024 high detail, or 1600x1200/1680x1050 at low detail. Considering the cost of the system, a 19" TFT will probably be the optimum complimentary purchase so this isn't too much of an issue, just make sure you try and go for a passive card, or at least one with a large/slow fan for minimal noise.