There is an option to enable single core optimization in the full game, that I don't know if the demo has. If the demo runs ok then the full game should do just as well.
Do you know anybody with broadband? The computer store may be willing to download the demo for you (I know of a couple that will download large patches and burn them to CD/DVD, at least); if you dealt with a particular salesperson there getting the computer, try asking him/her.
I have not looked at video card reviews or pricing in quite awhile, but you can often get a better price/performance ratio going one step up from the lowest end (ie a good increase in performance for not much extra money). I don't know if the 150 is low end or was just the lowest end compatible card they had, though, so that may be the best option.