Regardless of what you think of GameStop or BestBuy, this won't hurt GameStop because...
1. BestBuy won't price match GameStop's discount card, too.
2. BestBuy won't price match GameStop's coupons.
3. BestBuy won't price match GameStop's promos (buy2get1, etc.).
4. Most BestBuys don't take trade ins.
5. For the $2 to $5 difference, GameStop still more locations (sometimes it's cheaper than driving).
6. BestBuy's video game selection in-store is still pretty small by comparison.
...and the real big one...
7. BestBuy still won't take back the games if you don't like them (which GameStop does for used games).
Granted, some people will still rush to BestBuy, but every time someone used to buying used games with a return policy buys a piece of crap game they can't return at BestBuy, they'll abandon BestBuy to return to GameStop.
(I didn't talk about GameCrazy because I've never been to one and I'm unfamiliar with their prices, policies, etc.)
Last thought... I know a lot of people on here hate GameStop and a lot of people on here hate BestBuy (in either case, it's almost always due to an idoit or two at a local store for one chain or the other), but how much does that affect your decision to buy when policies are better where the idiots are?