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Limited only to the two shown, the tower model has the better processor and would therefore be a better performing unit. I wouldn't expect to see high frame rates even with lower settings but, I would think you should be able to get some playablility out of it. Small form factor units are best suited to the HTPC role IMO and really aren't meant to be 'gamers'
Hope it helps
The first Pc is definetly the better of the two since the cpu has a higher stock speed and 2gb on the video card.
You do however want to look at the system requirements of the games that your looking to play to see where the computer stands as far as what you might expect for frame rates and resolution.
Below is a link for the graphic card hierarchy chart and as you can see by the chart the video card that is in both computers is pretty far down the list and may not make the minimum system requirements. If it does make the requirements it will be just making it and you will have to play at reduced resolutions and settings to get an acceptable frame rate.