If your internet sevice provider (ISP) is giving you a low/slow bandwidth connection, no amount of dongles or router upgrades is going to correct the situation. All that 300mbps dongle to router will do for you is make your small gap between ps3 and router speed fast, nothing more. Where you need speed is out to the rest of the world, and that has to come from your ISP.
If you had a 10/100 router attached to your ps3, but a 1.5mbps download speed connection, you're not going to get things done online very fast. But if you had that same 10/100 router connected to a 50mbps connection, oh yea. Much much faster downloads and smoother connections.