i want to upgrade but where to start

[Savage_Smithy]

I want to upgrade but where should I start?

I have a pc that i built around 2-3 years ago and while it plays games like far cry 3 ok it does go through random moments of really low fps. one second it may be at 30fps and then suddenly it will drop off to 5fps.

so im looking to upgrade it so it can keep up.

the current specs are:

mobo - asrock m3a770de
cpu - amd phenom ii x2 555 BE
memory - crucial ballistix 1600mhz 2x2gb
graphics - 2x gigabyte radeon hd 5770's in crossfire
psu - thermaltake evo blue 650w
will soon have a 128gb sandisk ssd for the os too

so, since im on a limited budget i will have to upgrade 1 thing at a time, so should i upgrade the cpu/mobo first or the graphics cards?
also the old parts will be sold so what should i be asking for them? ( i know they aren't worth much)
all in perfect order and have boxes
everything else is okay i think, though i may add more RAM

I have just realized that if i upgrade cpu/mobo first i would probably have to get a motherboard capable of crossfire so i guess that would work out more expensive. Still though I don't know what to do.

thanks,
smithy