yeah, go for it - i mean if them computer vendors understand their business they will gladly sell you the hsf of your choice and install it for you. better leave it to the pros - might be a good idea for you to watch how it's done - it's no big deal really, so next time you can just do it by yourself and see how much fun it is
have you searched for arctic cooling hsf? afaik there are plenty of various models for particular chipsets so maybe one of them fits a reference 7600gt - ppl say the arctic coolings are both efficient and quiet - might be worth a try. they're pretty big tho, so better see if your northbridge or hdd is not in the way (i had a little problem with that but i just repositioned yje nb cooler and it's ok now)
yeah, definitely make sure if your card and the new hsf are not too long - this could happen if you have a short or cramped case
my experience is - do the math (measure it) before you buy it, not the other way around