Looking to upgrade my old desktop: Budget <$100

[mops]

So I'll be using this PC to play games like CS:GO, DotA2 casually PC specs are -

E2160 @ 1.8 ghz OC'd to 2.4ghz processor
2gb ddr3 @ 1333 mhz
Gigabyte G41-m combo motherboard
Palit geforce 8400 gs 512mb gpu , stock 567 mhz oc'd to 720 mhz
400w stock psu


Currently I'm barely managing 30 fps in DotA2 and 45-50 fps in some CS:GO maps, inferno nuke etc are basically unplayable.

My question is which upgrade would directly increase a huge amount of fps in those games, I can afford either a CPU & mobo OR just a gpu, since the cpu is LGA775 i'm worried that a new gpu would get bottlenecked because of my cpu.

I also don't have any cpu/mobo or gpu shortlisted to go with that setup so any suggestions regarding that would be great too.

Thanks.

 

[clutchc]

To get the most gaming pleasure for under $100, I'd upgrade the gfx card.
http://www.amazon.com/ASUS-HD7770-DC-1GD5-V2-Radeon-GDDR5-Graphics/dp/B007RFYFH0/ref=sr_1_11?s=pc&ie=UTF8&qid=1377721922&sr=1-11&keywords=hd+7770
...assuming you have a big enough PSU.

If not, look into the HD 7750: http://www.amazon.com/HIS-H775F1GD-Displayport-Express-Graphics/dp/B007827196/ref=sr_1_9?s=electronics&ie=UTF8&qid=1377722116&sr=1-9&keywords=hd+7750