Im pretty new to these forums so i wasnt too sure if this was the correct board to post , but no doubt i'll find out soon enough

My problem is that i cant run any newer games , even some of the older ones like black hawk down , splintercell series , quake 3 , rtcw ,
to name a few.
I try running some of these games on the lowest settings and yet i get a really bad frame rate and can barely run them .. i know lots of people that have the same sort of system specs or lower , yet they get much better results when playing games .

Im not sure what it is thats causing my system to run very slow , i upgraded my gfx card a while ago and my hdd but nothings worked so far , i also bought a new case with plenty of cooling but that didnt solve my problem , im not sure on what motherboard i have but i was told that it only runs my gfx card at agp 4x instead of its 8x which is good or bad ?
heres some of my system specs if they are of any help :

Athlon 1800xp
Pixelview Geforce fx 5900
512 ddr ram
Western Digital 120gig 7200rpm
400watt psu

i was looking into upgrading my motherboard and cpu soon , would these be good choices ? and maybe boost the performance of my gfx card/system a bit ?

Cpu -AMD Athlon 64 3500+ 2.2GHz Socket 939
Motherboard -MSI K8N Neo Platinum nF3 250GB