"Silent Hill series" (there are 4 titles, silent hill 2 is really amazing) (can play on onboard graphics)
"The Suffering series" (2 titles, both rocks) (need an external gfx card e.g.FX5200)
"Doom 3" and its mod "Resurrection of evil" (fast paced action, u got to be a sharpshooter with good reflexes) (needs an external gfx like FX5200 )
"The Thing" (Old but graphics and screenplay are one of the best, watch out the movie also) (can play on onboard graphics)
"Return to Castle Wolfenstein" (id softwares incarnation to amazing wolf3d series, good scary game but can be tough at times) (can play on onboard graphics)
"Clive barker's Undying" (good but u may feel it a rip off from unreal if u hav played too many unreal engine based games) (can play on onboard graphics)
"F.E.A.R" (just get it if u hav a nice external graphics card)
"Condemned: Criminal Origins" (one of the best melee combat game i ever seen, its the latest one to play) (coz its based on F.E.A.R engine so u will definitely need an external gfx card e.g. GeForce 6600 or7300GS)