Yeah, gotta agreee with brinks.
Socket A's are great, but they are a bit long in the tooth.
If you want to build on a budget, snag a 64 bit when the price drops in a couple of weeks.
Get a decent mobo... ABIT KN8 Ultra or Foxconn NF4SK8AA-8KRS Socket 939 NVIDIA nForce4 SLI or so.
You can use your current RAM for now, as well as your hard drive and optical drive(s).
Grab a decent graphics card (PCI-E) eVGA 256-P2-N562-AX Geforce 7900GT
Get any case that catches your eye, but a good PSU (Antec, 500W+)
Once you have that, you can take what's left over from the $700 and invest it in some faster RAM and a SATA drive.
My Rig: SABLE
Antec 300 Illusion / Antec EarthWatts EA650 650W / EVGA GTX 760 2GB
AMD FX 8320 x8 Black Edition / Gelid Tranquillo / MSI 970A-G43
Sandisk Ultra Plus 128GB / Samsung 840 120GB / WD Black 750GB / WD Green 1TB / WD Red 3TB
G.SKILL Ripjaws 2x2GB DDR3 1600 - Crucial Ballistix Tracer 2x2GB DDR3 1600
Win7 Ult 64-bit - Func MS3 Mouse - CM Storm QuickFire Rapid Mech Keyboard
R.I.P. Danny L. Trotter
... 14 Nov 1945 - 4 Sept 2009