I honestly don't see the point in upgrading to a new board to keep a quite frankly old and slow CPU. If you had a C2D It'd be different.
If you have £150 max then have a look at Scan, I'm in the UK and live down the road from them. They're not the cheapest, but they have excellent customer service.
I've specced some up for you here, comes to £144.50 without delivery but inc VAT:
2GB (2x1GB) Corsair XMS3, DDR3 PC3-10666 (1333) Non-ECC Unbuffered, CAS 9-9-9-24, 1.50V 28 day installation damage insurance with Scansure available on this product £49.90
AMD Athlon II X2 250, Regor Core, S AM3, 3.0GHz, 512KB x 2 Cache, FSB/HT 2000MHz, 65W, Retail £50.82
Gigabyte GA-MA78LMT-S2, AMD 760G, AM3, PCI-E 2.0 (x16), DDR3 1066/1333, SATA 3Gb/s RAID, MATX, VGA £44.03
For your budget of £150 that give you maximum upgrade potential if you consider upgrading next year aswell. Plus there would be not point you wasting money on DDR2 either as you'll only end up junking that on next upgrade.
And don't think I am an AMD fanboi, or that I think 775 is dead. I am far from that! I currently have a Pentium Dual Core myself and I made the mistake too of staying on 775.....