ok, i can't find out if your case will support a micro atx board. here are some sample builds but it wasn't easy.
If you want both the Athlon (pc part picker calls it an athlon II for some reason) and the HD 7770 and power supply....
http://ca.pcpartpicker.com/p/1iaLk
you could save some money by dropping down to 1333 mhz ram on that build or you could try salvaging your PSU. I'll include 2 more builds for you here.
and second....
http://ca.pcpartpicker.com/p/Jl6S
This build is just there to show another option. Going with 1866 mhz ram for this one is required. This build would be utilizing the power of AMD's APU, for which, faster ram = much better performance. This can hold you over until you can save up for a dedicated video card or you can try out the dual graphics (crossfiring integrated and dedicated graphics). The problem is that only works on with the HD 6670 and general consensus is that it isn't worth it. The HD 6670 would cost around 60.00 or so but still, not worth the small increase it offers.
as far as which one would perform better, the dual graphics isn't there yet. maybe if you could crossfire with a 7750 it would perform much better. It helps but not enough to justify the cost of the system. However if you find you can't afford the athlon build, you might do well with the A-10 and get a 7770 or better yet GTX 650 ti boost or HD 7790 later on.
I think you should consider the first A-10 build. Especially if you need to get a case (I'd be damn sure you can mount a micro atx board in your current case first before doing anything.) The desktop APU's are a decent solution to a problem like this. Seeing as how you are on an extreme budget, you can save the cost of a GPU for the time being and upgrade later.
If you are sure or you think you can save up for a cheap case that will house a micro atx board then the Athlon build i linked would be your best bet. You sohuld