Computer not working to full potential?

[Wasteblade]

I recently build my own computer (2nd one). I have all my old parts, which I put together and made a second computer. The one I have now has all the newest stuff, but lags while gaming. My friend, my brother, and I play GTA IV on LAN together. My friend was on my older PC, I was on the one I just built, and my brother was on one he just bought. I lag. Nobody else does. I tried to single out the problem, but I can't seem to figure it out.
The only thing I could think of would be our OS's. I have Vista 64 bit, my brother has 7, and my old one has XP.

Any help?


------------------
System Information
------------------
Operating System: Windows Vista™ Ultimate (6.0, Build 6002) Service Pack 2 (6002.vistasp2_gdr.100608-0458)
Language: English (Regional Setting: English)
System Manufacturer: MSI
System Model: MS-7612
BIOS: Default System BIOS
Processor: AMD Athlon(tm) II X4 640 Processor (4 CPUs), ~3.0GHz
Memory: 4094MB RAM
Page File: 1934MB used, 6430MB available
Windows Dir: C:\Windows
DirectX Version: DirectX 11
Graphics driver: 3 MSI N250GTS 1GB graphics cards

 

[Shadow703793]

Specs wise, you should be OK. HOWEVER, what resolution,etc were you playing at?
Btw,

3 MSI N250GTS 1GB graphics cards

Do you mean Tri-SLI????? WHY would you do that?!?!

 

[Wasteblade]

Is it bad that it's Tri-SLI? And I was playing at my monitor's highest which was 1920x1080


Edit: I usually use the third one as dedicated to PhysX.