Considering Upgrade, Motherboard, Ram + CPU

Hi,

I am currently considering an upgrade to my current spec, which is as stands: -

Thermaltake Armor+ Black Case - Massive Case, plenty of room for upgrades!
AMD Athlon 64 x2 Dual core 4400+ 2.20Ghz
Asus A8N32-SLI Deluxe Socket 939 Motherboard
Corsair 2GB DDR 400MHz/PC3200
XFX 8600GTS 256MB DDR3 PCI-E GC
Western Digital 250GB & 200GB HD
Sony 18xDVD±RW/RAM Dual Layer
Samsung 20" Widescreen Gaming TFT Monitor

I am currently trying to find a Motherboard, CPU and RAM for a price of upto about £500.

I was looking into getting an Intel Quad core CPU, compatible MB + Ram, bt i have been reading threads and posts on here for ideas and inspiration.

I have been an AMD person every since i had my 1st PC, bt atm i am looking into Intel alot more.

I will be using the system as an gaming rig

Possible OCing would be a must

Possible SLI, as i may consider getting hold of 2 new GC's in the future.

I am considering this upgrade towards the end of the year, but advice now would be awsome!

cheers