Why are you still listening to them lol. Those idiots won't know computers even if their lives depended on it (oh the irony)
Yes it is perfectly fine to upgrade your PSU, choose any PSU from these good brands (make sure it is at least 600W if you want a GTX 460, though a HD 7850 only needs 500W, but who doesn't want to be safe?)
How to know if the HD 7850 works on your mobo? Simple you need to know 3 things
-Does your mobo have a PCI e 2.1 slot (you most likely do)
-Is your PSU good enough (once you upgrade, yes)
-Do you have enough space to fit your card of choice inside (that I have no idea, please check)
This is about the PCIe slot.
I would say get XFX Core Pro 550W 54$ after rebate on ncix.
And about the GPU what is your budget ?7770/7850 Would be great ! Lower power than Gtx 460 and a bit better
Or the budget for both PSU/GPU.