Wireless Router G vs N Speed Question

I currently have a linksys wrt54g router for many many years already. My linksys wrt54g is still pretty good for my laptop that works good with the wireless. However, when i download something big. it rarely goes over 200kbps on the laptop to download something. But when i download something on my desktop it always goes at least 500kbps. Sometimes it goes 800kbps and 1mbps when...