You can buy an LCD 17" right now off of outpost.com on their sale for $199, if you want to splurge there's a 17" LCD on fatwallet.com forums right now for $250, that has the higher res & refresh rate, so let's say $250.
Motherboard plus CPU for athlon 64 3400+ on outpost.com last time there was a sale was $210 for the combo. (edit: $280 as per pricewatch for a mobo combo + processor for the 939, including fan & cpu, copy & paste: GIGABYTE GA-K8NS-939 & CPU AMD ATHLON 64(939PIN) 3200+ w/fan from PCinfinity.net).
Xblade case, with power supply, $61
120 GB drive, $59
1 GB Kingston memory going for $66.99 x 2 so ~ $134 right now, outpost.com sale right now
9800 pro video card, $270 brand new
-----------------
total cost brand new = 250 + 210 (edited $280) + 61 + 59 + 134 + 270 = ~$1055, which includes all manuals, etc..
Given that the equipment is used, and warranties etc may be lost, as well as original receipts etc you're gonna take a %40 price cut by buying it as-is, even if it's only a month old.
1055 * .60 = ~$633, or $700 given the work putting together etc.., that includes the monitor. Even adding a licensed legitimate version of windows XP pro, tack on another $89, making it $800 being overly generous.
$950 for that system used, you'd have to be a used car salesman to get someone to pay that much that was a fellow computer gamer, when you can get a package deal for Dell with a 19 " LCD and a faster system for cheaper. You might be able to get that price from a non computer person, but this is the wrong forum for that, for what you are trying to sell it for you aren't going to get that here.
----------------edit----------------
My bad, didn't see the socket 939 initially. Boost the mobo + processor combo price up to $280, as per pricewatch, so I tacked on $70 for the prices I gave.