Problems with gt240 nvidia

hello guys i just have bought a new graphic card and i have install the new gt240 card,i fhirst runned a 8600gt and now i am switched too a 240gt 512mb My problemn is if i start my computer there will only be a black screen and nothing happends i am running windows 7 but that wont be the problem bcs its supports windows 7.
The only problem i think is my comp use 2 gb RAM ddr2 800 and the graphic card is drr3.
my comp information:

amd athlon 64 6000plus 3.12 ghz
Motherboard: Abit NF-M2SV
RAM: 2 gb
Old graphiccard: 8600GT
New: gt240 Nvidia

PLS Someone Now A Reason Why The screen will be black?????????????