So, recommendations... Let's see: if you hadn't already you should try
HALO: Combat Evolved (pretty decent FPS SciFi, extremely fun and easy to play, with a wee-bit of depth),
Brothers in Arms (if you're into WW2 realistical FPS),
Deus EX (a highly praised, perfect mix between RPG and FPS cyberpunk),
Fallout 1,2 (post-apocalyptical RPGs Mad Max style),
Fable (if you like your RPGs a little more fantastic... orcs and dragons and such), the
Silent Hill series (if you have strong nerves and a healthy stomach
for the wickedness of japanese horror; although, SH isn't neither RPG, nor FPS, nor adventure. it's survival horror) and
Scratches (a modern adventure Myst-style horror masterpiece).
I guarantee that all the above mentioned run like a dream on your rig. I hope you'll enjoy them.