Wireless networking speed

Excuse me here, but I really know nothing about networking...

I bought a Linksys B router for my DSL modem and a Linksys B USB device for
my 600mhz computer. The results were terrible; it would take maybe 10 times
as long for most sites as the direct connection, and some sites simply did
not load.

I bought a new 1.8ghz computer and put the Linksys USB on it; and moved the
600mhz machine to my son's room (about 1/3rd the distance as before) and put
a Belkin G PCI card on it.

The 1.8ghz computer was significantly better than the 600mhz machine was in
its place, but still slow. The 600mhz machine (now with the Belkin) was
even better; nearly as good as a direct connection.

Thinking the Belkin was better than the Linksys (and Office Depot having the
Belkin on sale for $10) I bought another Belkin and put it on the 1.8ghz
machine. It is now as good as a direct connection.

So, what can I attribute my good results to?
1) Belkin is better than Linksys (Belkin is certainly much easier to
install)
2) PCI is better than USB
3) G cards work better than B (even with a B router)
4) Something else entirely

Thanks