What I'd do is sell your current PC, and use that money to buy RAM so you can run a copy of XP on that old pc. (1gb or more should be fine).
The computer you'd like to sell is probably worth anywhere between 700 and 900 dollars depending on the specific model numbers on the parts, not including the monitor. I have no idea what size monitor you have, so getting an estimate on that is impossible. All you gotta do is lookup what it's going for on ebay or amazon, look for the lowest price, and consider that the going rate.
I'd say you should try to sell it locally for 900 + the cost of the monitor and see what happens. I'd try listing it on craigslist, and on a for-sale group on yahoo groups for your local area if your city has a list like that.