Gaming PC advice needed.

Azza

In the next month or so going upgrade my current PC.
Primary purpose is gaming.
Want to be able to run games at high details (not necessarily with high amounts of Anti Aliasing or Direct X 10). But something that can handle resolutions around 1600x1050.

Games I will be playing will include but not limited to.
ET:Quake Wars
Unreal Tournament 3
Call of Duty 4
Crysis
World in Conflict
Company of Heroes/Opposing Fronts
Plus alot of Source Engine powered games.

My current machine has some annoying drawbacks. Like only 1 PCI slot which I can't use as the graphics card blocks the PCI slot, so no sound card. Worst motherboard I ever decided to buy.

Secondly my CPU is starting to bottleneck my machine on newer games an old socket 939 Athlon 64 x2 3800. 939 no longer has any upgrade paths so that forces a overhaul of CPU/Ram and Motherboard.

Finally my case is a rather cumbersome Thermaltake Thai Chi case so I think that needs to go as well.

Dual core or Quad Core?
Intel Core Duo or wait for the new Core Penryn or wait for AMD Phemons?
Decent motherboard and ram to match with the above processors.
2GB or 4 GB of RAM
What OS would you recommend XP or Vista(which version if vista)
Finally I'm on a Sapphire x1900xtx card which is still quite good. But thinking of switching to Nvidia. You guys reckon I should wait till the next generation of graphics cards or buy now?

Hard Drives, Mouse, Monitor and power supply (ocz powerstream 700w) should be fine for now.

I'm willing to pay for good value hardware. Overclocking ability is
of secondary concern to me but if an item can easily be overclocked I would not mind

zonEEE

In the coming months the new Nvidia cards are coming out and is the new intels so only time can tell O: Just wait for them to come out.