Unfortunately my board only supports up to 8GB (4x 2GB) of DDR2 RAM, this is what I'm looking at getting from eBay: http://www.ebay.co.uk/itm/8-GB-4x2GB-DDR2-PC2-6400-6400U-DDR2-800-MHZ-MEMORY-DIMM-PC-DESKTOP-RAM-240-PIN/221789630411, CEX will only let me buy 2 sticks at once.
Thanks for the advice, the 7850 definitely seems to be the better choice, I'll keep you updated with whatever I end up doing.