I think it could greatly benefit from a graphics upgrade, and with your choice of games, could probably be a viable gaming rig for a while yet.
HP uses decent PSUs, so your 300w should be able to most graphics cards that require 350-400w.
In that range, at your budget (100-150) A 5750/5670/GT240 are all solid choices. A 5770 would probably push the limits of the PSU, if the 5750 isn't already. A 5670 or GT240 would both be fine, though I would push the 5670 before the GT240, for its higher performance, even though the GT 240 costs less. Regardless of price, they are both well within your budget.
Might the money be better spent saving towards a new system in the future? Perhaps, depending on how long you want to wait. On the flipside, an investment in a video card is not lost, as it can carry over to the next system.