Hey hope you can recommend what computer would be needed to play the latest PC games on a medium to high settings. Say games like COD 4, Bioshock, Assassins Creed, Company of Heroes, Neverwinter Nights 2, Crysis etc.

Your help would be greatly appreciated can help me and other people not so knowledgeable on PC. 

Are you going to build one yourself or buy a customized one? What's your budget?

Anyways, if you're building one yourself, here a spec that Crytek themselves recommended:
CPU - Intel Core2Duo E6750
GPU - GeForce 8800GT 512MB
Motherboard - NVIDIA nForce 650i Socket 775
PSU - 600W ATX12V
RAM - 2GB (2 x 1GB) 240-Pin DDR2 800 (PC2 6400) Dual Channel Kit
HDD - SATA 250GB 7200RPM
DVD - 20x DVD±R Burner
Case - ATX Midi Tower Computer Case
OS - Microsoft Windows XP Home with SP2

Cost about $900. (I might make some adjustments to it, though seems a bit old)

Ok I just want to know whether my PC can run certain games.
I have a one year old PC with Windowx XP. It has 1024MB system ram, AMD 64 Athlon 3500, graphics card is a 7900GT Nvidia Geforce card. Will it play COD 4 or Assassins Creed on medium settings?

 CPU - Intel E8200 or E8400 - same price range as E6750 but faster and better

GPU  - 9600GT = better

 OS - XP Professional - best Windows OS ever

PSU - 700w - trust me it will allow expansion 

 Yeah, it'll play Call of Duty. For AC, the PC version completely sucks-Your CPU doesn't meet its system requirements.

 Would I need to put in more RAM say 1GB or 2GB. Or the the PC is just not up to scratch regarding the processor.

 see my reply to total totalwar's post, build a desktop using his list plus my changes and you will have a good PC.

Thanks guys I will look into it and have to see how much it will cost.

My XFX 7900GS XXX could run crysis on medium, never tried pushing it, unfortunately 3 days ago it decided to cook itself so I'm running on my old 7600GT.