what's a risers card??

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

Recommended: Intel Core® 2 Duo 2.2 GHz or AMD Athlon 64 X2 4400+ or better
You Have: 4 processors running - Intel(R) Core(TM)2 Quad CPU Q9300 @ 2.50GHz PASS
System RAM
Recommended: 1 GB Windows Xp / 2 GB Windows Vista
You Have: 3.5 GB PASS
Operating System
Recommended: Windows® XP/Windows Vista® (only)
You Have: Microsoft Windows Vista Business Edition, 32-bit (Build Service Pack 16001) PASS
Video Card
Recommended: 256 MB DirectX® 10.0-compliant video card or DirectX 9.0-compliant card with Shader Model 3.0 or higher (NVIDIA GeForce 6800+ / ATI Radeon X1600+)
You Have: NVIDIA Quadro NVS 290 () PASS
Video Card Features - Minimum attributes of your Video Card Video RAM: Required - 256 MB , You have - 1.7 GB
Video Card 3D Acceleration: Required - Yes , You have - Yes
Video HW Transform & Lighting: Required - Yes , You have - Yes
Vertex Shader Ver.: Required - 3.0 , You have - 4.0
Pixel Shader Ver.: Required - 3.0 , You have - 4.0

Ok well this is the test for the recommended requirements for assassins creed but when i go to play it it goes like really slow and i dont know why, i have turned down the reslution down to 800x600 and it is playable but still a bit slow, anyone know why is this happening and how can i improve? i dont know the fps because i dont know how to find it out. thanks