OK, what I found so far in regards to your system upgradabuility:
Processor Front Side Bus Frequency: 400/533 MHz
This means a max of a Celeron 340 (2.93 GHz) and a Pentium 2.8 GHz for a 533 FSB, which your mobo does support.
As for what PSU, your case should be a micro-tower, and your mobo is a micro-ATX. So, you have to get a micro-ATX PSU. Check this one out:
It has 450 watts.
The same goes for mobos. Your case is a micro-tower, so buy a micro-ATX. There are many good ones out there, but remember this... you'll have to reinstall WIndows afterwards, as well as everything else that is on your hard drive.
Your motherboard will handle an AGP version of the 6800GT.
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