An AGP 4650 is a good option to keep gaming on the old system if that is your only choice. It is one of the most powerful AGP cards available. (better is the 4670 AGP, and 3870 AGP--but they are too expensive) It should give you reasonable WOW performance on a medium/small size monitor.
(is it a good investment?--no--because it costs twice as much as its modern counterpart)
Yes--get DDR3 if its not too much more expensive. It will improve the performance of the card quite a bit over the DDR2 models. You dont need more than 512mb ram on the card so dont pay for that.
However-- my real suggestion for people looking for AGP cards is to check craigslist. Old used AGP cards are CHEAP. New ones are more expensive than their modern counterparts because of the low inventory. As long as you get a chance to verify that they card works before paying you should buy whatever used card you can find.