For a lightgun, I would use a sm750. Rangefull and compact. Can't go wrong. Plus, it may be the only airtank you can fit in there without it being homemade.
What the hell are you talking about? Mass effect works fine on vista. The tech specs are higher than crysis though. You might want to look at craigslist or ebay, nerfhaven really isn't for selling stuff other than nerf.