Problem is cards under ~75$ perform HALF as well or worse so they provide a terrible value.
Since AMD just renamed the 77xx cards they are all being cleared out under $100. You can basically get a 7790 for the same price as a 7750. I do see that your PSU does NOT have the PCIE plugs needed...but I still think it could run a 7790 with an adapter. A 7750 is like 70w (no plug needed) while a 7790 is like 110w (needs 1 plug).
Currently cheapest GDDR5 7750 card on newegg:
http://www.newegg.com/Product/Produc...82E16814131461
$70 after $30 rebate, but that cooler is a bit scary.
Here's the same brand 7770:
http://www.newegg.com/Product/Produc...82E16814131488
For the same $70 after $30 rebate with a bigger cooler
Another brand 7770:
http://www.newegg.com/Product/Produc...82E16814202011
$80 after $15 rebate
This is a 7790 that was on sale, but sold out:
http://www.newegg.com/Product/Produc...82E16814131490
Was $70, then $80 after $20 rebate.
A 7790 is about 30% faster than a 7770 and 60% faster than a 7750.
Your CPU wont use more than 50w, add 5w for the SSD and another 5w each for the MOBO/DVD drive ect and for the RAM. Your system right now is not going to use more than 100w MAX. Even if you added a 150w card you're still not going to be crushing your PSU. Antec is respectable enough to give fairly honest ratings even on their budget line...perhaps it can only give you 300w continuous, still enough to drive a 7790 (with 2xMolex to 6 pin PCIE adapter).
Normally I wouldn't suggest going as high as a 7790, but since they can be found for so cheap now there's no reason not to grab one. Just keep your eyes peeled for a sale.