Is upgrading worth it?

What resolution do you play at? A 4670 would almost certainly suffice. Either way the 650TX is overkill, the 450VX will handle a 4850 well, a 4770 and 4670 both use substantially less power.
http://www.newegg.com/Product/Prod [...] &Tpk=450vx
http://www.newegg.com/Product/Prod [...] 6814102854

the gaming performance increase over the GMA is definitely worth it, 3100 is total crap, if you check this link you will see the 4670 is 9 rows from the top while the 3100 is 3 from the bottom, that is a massive performance increase
http://www.tomshardware.com/review [...] 404-7.html

A 4850 would easily max out that resolution, especially on the games you intend to play so there is no need to consider high than that. Also your motherboard only has a single PCI-E slot so you dont have an option to SLI or CF in the future, if you want a PSU that will handle any single card setup then get the 550VX.

Not enough to justify $30, 5% at best, however most of that price difference is due to the extra vram on the 4850 which you wouldnt utilize anyway. The 4770 is finally down to the price it was supposed to be which is very good, make sure you have enough airflow in your case though as i know HPs tend not to have quite enough fans but yours may have intake vents which would be helpful. The 4770 is probably your best option so that you can have something that will exhaust the heat but it will need decent intake air so that it has something to exhaust, if your case doesnt have enough airflow the card might run hot, but a few minutes with a dremel and you have yourself an airflow improving case mod.