My imac is runnning on the 5ghz channel at 120mbps but my second PC is running at only 54mbps...!
How can I change it to run faster as I am getting very slow speeds from the PC compared to my mac. PC is showing running on the 2.4 ghz channel.
PC is getting 8mb DL and 12mb UL
iMac is getting 65mb DL and 16mb UL
Wireless N does not mean 5 Ghz automatically. If you go into the wireless adaptor properties, if its capable of running 5Ghz you should be able to selected it. If not then it seems like you have a 2.4 Ghz wireless adapter only
How much does a wireless USB dongle for 5ghz cost?
They range in price from about £15 to £35. Do a search of a well known online shop and I'm sure you'll find them.
You could also consider changing your PC's wireless card if you are confident in opening up your PC and changing components. They range in price fom about £25 to £40. The USB adaptor will be the easiest option to fit but the PC wireless card will probably give a better range because they come with better aerials.
If you do decide get either of them make sure that it is capable of dual band so that if you have problems with 5GHz you can revert back to 2.4GHz.
One like this would suit your needs: Linksys WMP 600 N Wireless-N Dual Band Adapter.