Is it worth upgrading my GFX card in this situation?

[Milites]

My current spec is:

Intel Core duo 2 quad @ 2.4 GHz
305w psu
AMD 7770 gfx card
4 GB ddr2 ram
64 bit win 7 home prem.

I mostly play WoW and other mmo's... I rarely (and wouldnt be too bothered if I couldn't play them till next upgrade) play games with super high requirements like battlefield, gta etc and have the option to upgrade my graphics card to a 750 ti tomorrow (can get a new psu if needed).

or should I wait till next week and get a I5 + new motherboard and ram + keep my gfx card... link to exact model - http://www.techpowerup.com/gpudb/b72/his-hd-7770-iceq-x.html

How would gaming be on a machine with the specs of:

Intel core duo 2 quad 2.4 GHZ
GTX 750 TI
4GB ddr2 ram