by saying 300Mbps, I am assuming you are wanting an 802.11n card.....
Before you run out an buy an 'N' card, what have you done to troubleshoot the issue?
-How many hosts (computers/game consoles/etc.) total on the network?
-Have you tried powering down all the other hosts to see if the trouble persists?
-What kind of connection are you on, and what speed is it?
-Have you ruled out enveonmental factors that would interfere with the 2.4GHz signal?? (use google, examples are microwaves, baby monitors, blue tooth devices, cordless phones are the top culprits).
-Does your current support 802.11n standard?
-If you do purchase an 802.11n card, will all wireless hosts be using the n standard, or are there a/b/g hosts? N standard doesn't play well with others in mixed networks I have seen reviews on.
-Also, n is NOT a finalized a standard, it is a DRAFT!!! This means that if you buy n stuff now, it might not play well with finalize n equipment in the future...
If you want suggestions on a card to buy, you will have to give us more details..... What kind of compute is this for? Desktop (PCI, USB) or laptop (PCMCIA, USB), what model router do you have? Till you trouble shoot, and give more details, I would recommend saving your money.