Well between those two, go with the 4500 hands down. And thats not because this is a DGL-4500 forum. Any 4300 you pick up will be used, since Dlink discontinued them some time ago. Which means no Dlink support or firmware updates. Also, 4500 has N, which is a huge step up. 4300 does have some features the 4500 doesn't, like wireless only mac filtering, but thats no where near the trade off. Don't get me wrong, the 4300 is a AWSOME router, but it just can't compete with the hardware advancements the 4500 takes advantage of.
With that said, It's important you look at the big picture. If the reason your Xbox is lagging while using netflix on PS3 is because your pushing your bandwidth limit, then no router will help. A HD netflix movie and a few PC's doing casual web browser can eat up a 5down/2up connection quick. What speed do you pay for from your ISP?