I duuno, IMHO It's really not worth putting money into your system.
I would just play those campains you can with FS2 vanilla, and other FSO campains with all the options off.
Your getting 20 bucks a month right?
So in one year, let's say you'll get 240$. I'll push that limit a bit, because at this point, I wouldn't recoment anything less for the sake of future upgrades you may wish to make, and bang for your buck.
OK, first off, go take a look at AnadTech's weekly buyer guide for an entry level system. It's not going to be able to handle the latest and greatest, but if you don't have any cash and you really want to upgrade your computer it's a good place to start.
http://anandtech.com/guides/showdoc.aspx?i=2159&p=1Check out these prices from the last page of that article (they include shipping costs):
-CPU & Cooling AMD Athlon XP 2500+ (cooling included) $86
-Motherboard ASUS A7N8X-X (nForce2 400) $65
-Memory 1 X 256MB Crucial PC3200 $47 (double that 512 = ~94$ or a bit cheaper if you get 1 DIMM)
Throw in a Radeon 9200 (if you want to play games DO NOT go for SE cards with cut memory bandwidth) for 54$
You need to get a new power supply with that, so I'll throw in a ballpark figure of 50$ [For a quality one - don't get a cheap one.]
That's ~$302 with 256MB ram, CPU, Graphics Card, MB and PS. Maybe you can get your source of income to front you some money, and pay it off. ( or do some odd jobs for people to get some cash. Lawn mowing helped pay for my first computer. )
If you shop around, I'm sure you can get some good deals. You'll definitely want the best vidcard you can get, and as much memory as you can afford. If you can, go for a CPU with a 333FSB (if you go AMD, which, on a bugget, still captures the best. YOu may wish to look at the AMD Semprons, their budget line, which are basically Athlon XP's with the same cache as the Thouroughbreds 256k)
Here's a good place to look (if you're in the states that is):
http://www.newegg.com/Cheers!