I am upgrading/building a new system, not blazing fast, but something that will not cost a fortune, but will cope with most games.
I propose using either a AMD Athlon, Dual or Quad core CPU, initially a Athlon Dual core 6000, but perhaps later upgrade to a Phenom. Also a GForce 8600 or 8800 Graph Card, 600W PSU, & 4Gig Memory, Just depending on the total cost. I have a case, HDD's, DVD drives etc
I am very unsure of the Mobo though, I find the spec's for AM2 & AM2+ boards confusing, as to if they support both dual & quad core CPU's. I would prefer to use a Abit or ASUS board, but I don't wish to spend more than £50 / £60. The ASUS M3A looks good but has been given some bad reviews, the M2N boards are unclear as to if they will support both Dual & Quad core CPU's, I like the look of the Abit AX78 best & it's about the right price for me. Can anyone give me some advice, as the Mobo is the heart of the system I would like to get it right

use a quad core.

Well I can tell you that you'll want an AM2+ board so that you can get dual core now and upgrade to quad later. It may also be worthwhile to get a board that already has PCI Express x16 2.0 slot(s). Not just PCI Express x16, but also 2.0. Also you will want it to support DDR2 1066, and Phenom FX CPU's if you'd like upgradeability to Quad Core later.

If you want my honest opinion, you'll want a board with a 790FX or 780G chipset or later. The new 790FX chipsets will have the new SB750 Southbridge and the 780G chipsets already have the newer SB700 Southbridge. Of course, the new 790FX boards will probably be $115 and up. However, the current 780G boards are as low as around $70 and the highly rated Gigabyte 780G board is around $90. There's the new 790FX and 780GX boards coming, but they may be out of your price range and I'm not sure when they will be publically released.