Re: cheap pc upgrades
Unfortunately, at that price point, it's very hard to get something with a lot of upgrade potential that you don't build yourself.
The things you want, in any potential PC:
Standard ATX case. Don't buy a "miniITX" or "MicroATX" - the MicroATX is OK as long as the rest of this is good as well.
Expansion Slots: Minimum of one PCI Express x16 slot. PCI Express 2.x preferred over 1.x, but both are fine
Memory Slots: Minimum of two, which usually isn't a problem. 4 if you can at all help it
CPU: Even if it's not a screamer, make sure you can drop one in later, that is. (Minimum AMD: Socket AM3 or AM3+(AM3b), Intel: Socket 1155 (NOT 1156)
Hard Drive, CDRW/DVDRW - doesn't matter too much, they're pretty universal
Power Supply - If it's a MicroITX case, they limit you pretty badly here. Don't even consider the system if you can't "size" the power supply with it being at least 5 inches across in it's cavity. Reference photos online of an ATX power supply to see what i mean.