As someone previously mentioned the $500 "limit" is probably just for cash withdraws and should not prevent you from using it on this purchase (assuming you have the fund).
As for using a credit card... There is nothing wrong with it, but as someone else stated don't do it if you don't plan on paying it off right away or if there is no "interest free" period. If you are going to get charged interest at all for the purchase then it's not worth doing it. I bought my rMBP with my Discover card mainly so I could get the extra 5% cash back, and then paid off the balance on my next statement.
In regards to buying such an expensive item for your girlfriend, that's none of my (or anyone else's) business. You know how your relationship stands (or at least I hope you do) and whether or not such a purchase is prudent.
As for which Macbook you should go with... Right now your best bet is probably a 13" MBP since you stated you did not want to go with the Air. The rumored 13" rMBP may be announced/launched before Christmas, but I doubt it will be around your price range. If she's a student you can also buy from the Apple Education store which will save you some $ and you don't even need a .edu address to confirm or anything.
