Perhaps this won't mean much to you, but you really need to test your LAN throughput to determine if wirelessly you can take advantage of higher speeds.
What I mean is, just seeing what speeds you get now between wired vs. wireless doesn't really help. What I am saying is, with one computer connected wired, and one connected wirelessly, you need to test the LAN speed between them, i.e., the speed at which data travels between those computers.
For example, I have a computer wired and one wireless. I copied a file from one to the other over my home network. I noticed I was getting about 13 megaBYTES (NOT megabits) a second. What did that tell me? It told me that that I'd quite alright getting the new 75 megabits down because 75 megabits down = 9 something megaBYTES a second. My wireless computer will get the full download speed no problem.
Now 150 megabits might be a different story.
I assume your computers are windows computers. If so, there is free software you can download to test your LAN speed. Once you know the speed between your devices on the LAN, you'll know what you can handle.
BUT having said all that, the router you are using now is a g router, 54 megabits per second is theoretical max for 802.11g, but we all know we don't get those theoretical max outputs. I would guess you wouldn't benefit from those higher speeds. Since 15/5 is too low as you know (and too expensive now in my humble opinion), you probably should get the next cheapest one, 50/25.
