HD7750 will give you better performance at lower power than the HD6670 (55w vs. 63w). Downside is higher cost ($100CA). http://www.hwcompare.com/11734/radeon-hd-6670-oem-vs-ra... It looks like your case takes a normal size video card (not low profile) and that will save you some $$ when you price out your new card.
250W PSU should be plenty for a 63W HD6670 plus your 65W Athlon II X2 215. Even if you OC the video card you should be fine. The HD7750 would give you a bit more power headroom, but the 250W is still plenty for the HD6670.
You will see a huge difference with the HD6670 vs. your current 6150 SE Integrated Graphics.