Should I rebuild my system or just upgrade?

here is my question,, I have a pretty decent system..which cost me alot of $$$ when put together cpl yrs back, all items bought at newegg...most or all of my hardware is outdated. have had advice to sell my cpu in parts..or just upgrade mem..gpu, and psu,,,for now..for me money is a issue..I like my system..it plays all max requirements on most games..even chysis..( which I don't have..or never played)..im not much of a gamer anymore..and only will probably play..games like madden 08, vegas 2,and might get modern warfare, H.a.w.x.,,and nba2k9, now my question is if I play these games with the system I have now..playing online,,if upgrade. gpu..mem..psu,,will it make that much of a diffence playing the games..or should I just save the money..buy a p-3 and just turn my cpu into buisness use..somthing I have been toying with alot..any help or advice would be appreciated..thanks..here are the specs of my syst.

1 x ASUS Striker Extreme LGA 775 NVIDIA nForce 680i SLI ATX The Ultimate Gaming Motherboard - Retail
o 1 x EVGA 768-P2-N831-AR GeForce 8800 GTX 768MB 384-bit GDDR3 PCI Express x16 HDCP Ready SLI Supported Video Card - Retail
o 1 x Seagate Barracuda 7200.10 ST3500630AS 500GB 7200 RPM SATA 3.0Gb/s 3.5" Hard Drive - OEM
o 1 x Intel Core 2 Extreme QX6700 Kentsfield 2.66GHz LGA 775 130W Quad-Core Processor Model BX80562QX6700 - Retail
o 1 x OCZ GameXStream OCZ700GXSSLI 700W ATX12V SLI Certified CrossFire Ready Active PFC Power Supply - Retail
o 1 x CORSAIR XMS2 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model TWIN2X2048-6400 - Retail
o 1 x SAMSUNG SpinPoint P Series SP2004C 200GB 7200 RPM SATA 3.0Gb/s 3.5" Hard Drive - OEM
o 1 x LG Black E-IDE/ATAPI DVD-ROM Multi Read Drive Model GDR-8164BK - OEM
o 1 x Microsoft Windows Vista Home Premium 64-Bit 3 Pack for System Builders - OEM
o 1 x Ideazon Zboard Gaming Keyboard - PW1USE1-B3ZBD01 2-Tone USB Wired - Retail
o 1 x SAMSUNG 204BW Black 20" 6ms Widescreen LCD Monitor with Height Adjustments - Retail
o 1 x Logitech MX518 8 Buttons 1 x Wheel USB or PS/2 Wired Optical Gaming-Grade Mouse - Retail
o 1 x SAMSUNG 18X DVD±R DVD Burner With LightScribe Technology Black IDE Model SH-S182M - OEM
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ok..so far..but will the 4 gig..give me alot more memory and speed? all around

cool thats sounds good..also noticed on some gpu,s they have hdmi hookups..does that mean u can hook up your card to say a visio tv.;and still get the sound from your cpu speakers?..