The 5770(or GTS450, i do not have one to test, but it looks good) is a great card for causal gaming. It plays most of the new games with ease as long as the settings and screen rez are not pushed too hard. Many games will do 1920x1200 with just slightly lower settings too.
If the rest of the system is up to it, you may be able to overclock the cpu a bit and gain some free speed.
Is that a Q6600? If it is, 3.0 should be an easy OC. Just make sure you keep an eye on the temps