A 2.5ghz dual core is kinda weak by today's standards. There are a good number of games where it will limit the GTS 450 at that resolution. If your motherboard allows for it I would definitely overclock that processor. Even just on the stock cooler 3.3-3.4ghz should be easily attainable which will make the processor much more passable for current games. With good aftermarket cooling 3.6-3.8ghz should be possible.
Also get 2 more GB of ram. It will definitely help and ram is very cheap at the moment.