If your looking for a good card for around $100 go for a Geforce 6200. There about $100 on newegg and they can handle halo and pretty much all the new games out right now with the appropriate settings. It even has a built in overclocking utility where you can increase cpu and memory clock speeds and unlock more pixel pipelines. After overclocking this card actually runs as fast as a 6600GT! If you want to spend a little more u can just buy a 6600GT there down to about $130 now i think, maybe less.
Nah. a something 600 card will knock the socks off a 6200, particularly one without dedicated memory. I'd say 9600/X600 at the least.