Chrismtas is coming and I thought about upgrading my system a bit. My current setup is:
Asrock P67 Pro
8GB 1600MHz G-skill RAM
Gigabyte GeForce GTS450 OC 1GB
128GB Crucial M4 SSD for OS and few games + HDD for data
Win 7 Home 64bit
Dell 2209WA 1680x1050
I use it mostly for Lightroom, Photoshop and gaming. Things like Starcraft 2, Tribes:Ascend, RAGE, Skyrim, Witcher 2, Dead Space, Company of Heroes etc, in general mixture of FPS and RTS games. Most of them work fine but sometimes I have to lower details to get around 50-60fps like in Tribes.
My options now are: i3 3220 (or spend a bit more on lower end i5) or graphics card upgrade. Is Physx worth getting another Nvidia card for it, or better stick with ATI 77xx or 6870 for lower power consumption and heat/noise? I like to keep my PC quiet when working at night, already got quiet CPU cooler and my current Gigabyte card runs almost silent with two fans on low speed, so I'd probably avoid 6870 for that reason.
Your biggest bottleneck is your graphics card. Your CPU should be fine although don't expect to get really high framerates in the most CPU intensive games. I would suggest you save up for a few months and get an i5 if you want to upgrade your CPU later.
The closest comparable NVidia Card is the GTX 660 which just slightly beats the 7850 but averages about 30-50 dollars more.
Since the GTS 450 requires a 6 pin PCIe power connector, either the 7770 or the 7850 should be able to plug in and work. However, without knowing what PSU you have, it's kind of hard to tell for sure so what is your PSU make, model, wattage and available current on the 12V rail?