They are compatible, but you'd be wasting money getting GT 610 as it won't be any more powerful than integrated graphics. If you told me your budget and what your son wants to do with the PC, I could recommend a better choice.
I have got a new case for him its huge but the psu goes in the bottom and the 20pin cable wont reach as the connector is at the top of the case is there an extension for that cable i could get or a psu with longer cables than standard? will the hd7770 be ok with this old mobo as it dont support pci-e 3.0? Please just indulge my questions i know im thick when it comes to computers and i do appreciate your help. Thanks mate.
Will the Celeron E3200 @ 2.4ghz drag down the GPU too much so it won't be able to play wow or will the GPU carry it? i've probably typed that entirely wrong but im sure you get the gist of what im getting at lol
Edit : Just read this comment on that power supply buddy : Good, tidy PSU, nice heathy voltages, all within spec. Practically silent at idle. The only gripe is the ATX power lead is a little short for reaching the other end of the motherboard, and the length of the 4-plug SATA lead could pose problems if you've got a lot of drives. Would be nice if it came with black screws, to go in my black case, but can't have it all I guess..
That's actually a very legitimate concern. What you're think about is called "Bottlenecking effect". Basically, how the computer games work: their code is executed on the CPU and then the CPU instructs the GPU what to do. In case CPU is too slow to do that, GPU isn't used up to its full potential.
To be perfectly honest, the card will be dragged down a bit. However, World of Warcraft will certainly be playable. The only thing he will need to do is reduce CPU intensive settings, such as shadows or draw distance, and minimize the use of 3rd party addons. In addition, the higher resolution and higher graphical settings one is playing, the more work the GPU has to do, while the work that has to be done by the CPU doesn't change. So increasing graphical settings, such as spell visuals, ambient occlusion, antialiasing or resolution, will not reduce performance and will instead increase the amount of card potential that is used.