hp dc7800 small form factor

hi there, ive got an hp dc7800 small form factor, its pretty good, core2quad @ 4x2,4GHz , 4 gigs of RAM memory, but the graphics is an nvidia quadro nvs 290 with directx 10, shaders 4,0 and it uses part of the ram so it has somewhere around 1 gig of graphics ram ( the gpu itself has 256mb), the problem is that although srtest.com says i overcome recommended pretty well, when i run games they dont look as nice and the fps arent really hight, i cant put any game to its max settings and then playing it smoothly. I thought it might be the gpu but i cant find any other gpu that can run with my pc, or otherwise, is it possible to install a normal gpu and playing without the pc's case? i mean playing it with the open case as if i put the gpu, i can't shut it, alsow im scared about getting a gpu and not having enough psu for it, can i change it too? thx a lot

what's a risers card??