i need your help folks...,koz i have gtx 275 on my rig,but still cant get smooth fps

[gtxlover]

this is my pc spec:
-cpu amd athlon X2 5400+ 2.7GHZ(stock cpu cooler)
-memory ram..4gb corsair ddr2 800MHZ
-galaxy gtx275 triple-fan
-AcBel 660W(high performance)
-motherboard ASUS M4N78
-casing cooler master elite
-basic hard disk and dvd writer

So here's my problem:
-i cant play gta 4 at medium settings,it will lag
-i cant get smooth fps playing far cry 2,but still playable at ultra high
-i cant play saints row 2 at low settings
-world in conflict will lag if there are many tanks and explosion on the
screen on very high settings
-left 4 dead is not smooth at high settings as it should be...when i pan
around,theres a little lag

So im not satisfied with my pc performance,
some say that i should change to quad core cpu?
or should i just buy a gtx295?
folks...can u help me please?what is/are my pc's weakness?
and does pirate operating system affect gameplay?
and does pirate games lack performance in fps?
and should i change my old amd athlon x2?

[marko94]

for gta iv u need to download all patches because it laggs on every system in its basic version ... and for far cry 2 its nothing unusual because that game is very hard to run in completly smooth fps as well as crysis