no its not overkill
this is your business, $300 extra on hardware is dirt cheap...
let me as you, are you manually trading or are you going to run any execution/algos? you buying breakouts? you scalping? what is your trading style? how bad is 20ms lag, how bad is 200ms lag, how bad is 2000ms lag to you?
if you are an active trader, your systems performance is critical, if you are are buy and hold, its less important, but either way, the difference in price going from a cheap chip to a 9450 or 6600 is a couple of $100 dollars...
to give you my background, i am developing algos for a large bank on the FX (and soon to be fixed income side) and then trading them...i am in the tech ramp up stage (i am not really tech oriented, more math/finance) but we spent at least 50,000 on hardware for servers, storage, etc....obviously this is not my dime, otherwise i would be on a lot cheaper (dont even ask my software costs, im guessing 400,000 right about now, for all in)....but this is your business and if you are day trading equities, i bet you trade 9-11am mostly (give or take)...thats 2 hours where you make your money or lose it....you dont want slow pcs messing it up when its only a few $100 more....