Just to let you know, my friend a while back got a pretty solid comp with monitor for 100$ (and the specs are > yours).
Basically he checked the newspaper, often times when schools upgrade (universities) or even companies like law firms, they'll basically take what they can get for the old comps.
You can either tell them your budget, or tell them what you are looking for, and they'll do their best to find what you're looking for. (They also typically prove the computer works before buying)
The only downfall was the comp that he got was in one of those "office" cases that are very small, bigger cases are better typically as they allow the hardware the most breathing room possible and also give you the most available room inside to work with.
But I mean, he got a p4 3.0ghz with monitor (big 21" CRT but still a nice throw in) that had a dece amount of ram, a good cd-rom and an alright hard drive all for 100$.
So rather than upgrade, if you're really strapped for cash, I seriously suggest you look into one of these options. My gf still runs a p4 3.0ghz and has no problems running apps, browsing, downloading and even being a backup pc for my World of Warcraft game. (It runs fine with a garbage videocard)
Didn't notice this post early as I made my post but then had to go do some work prior to finishing reading the thread.
1000 or less = very nice computer if you build it yourself.
However, if you know a person whose decently good with computers, help them browse with you at bestbuy or one of those places for a desktop at around 500-600 (everything included) with a good processor, harddrive, ram, etc. Then my advice to you, spend the other 400-500 on a brand spanking new power supply (a solid high watt one) as well as a very good videocard, and have said friend install the after market parts.
One thing I will say though, if you don't game at all, forget about the videocard/power supply and just buy a solid PC straight up for like 500$.
If you link us some comps you are interested in via make/model # I have no doubt the people here will be more than happy to give you advice on which one to get, or what the advantages/disadvantages of each are, or even which makes/models to avoid.
My problem with both Tiger Direct and Lenovo is that they have nothing against giving you trash power supplies. I remember a quad core system Tiger Direct was selling with a generic 250w power supply, and I opened up my cousin's Lenovo a couple of weeks ago - generic 280w power supply. One is definitely tempting the fates with power supplies like that. I BELIEVE that Dell on the other hand is supposed to generally use decent PSU's.