New entry level gaming pc build

[Zico]

Budget for the below is ~£350.

I've been out of the self build crowd for 3 years now but I'm looking to do a few for Windows 7 release.

I'm looking to do a deal with my younger brother (with conditions) to build him an entry/medium level gaming PC that has enough room for expansion when he gets his own cash.

I am looking to overclock the system so the best value CPU will be the focus of the whole build.

We need the following:

CPU - AMD/Intel no preference but value for money when overclocking.

Mobo - As above, no preference but good to overclock with some room for future expansion (CPU wise).

RAM - To match the mobo to overclock. 4 GB Dual or 6 GB Tri Channel.

PSU - To handle the above with room to handle an upgraded GFX. Preferably modular though not essential.

We have the following:

GFX - Looking to re-use a spare Leadtek Nvidia 7800 GTX for the moment and allowing my brother to save for his own GFX upgrade.
O/S - Will either use existing XP Pro retail licence or W7 RC until the October full release.
Keyboard/Mouse – Re-use existing
Monitor – Re-use existing screen
Case – Re-use existing steel tower