Does your PCI card have a high gain or standard antenna? The antenna in a laptop is normally similar to that of a high gain, as it uses the outside of the LCD panel as the antenna, so it has a large area for your signal to transmit and receive on. What is your signal strength on both systems? Also, what does the wireless card actually say it is connecting at on both systems? On my laptop it shows 37.9Mbps, it's very random, and constantly changing.
Also, if you have a lot of metal/cables around the antenna on the desktop, look at moving it away, or positioning the antenna so there isn't much metal/live wires between it and the router.
I would probably say, if your signal strength is low, go ahead and get a higher gain antenna instead of a USB adapter, as those adapters just suck.
If you dont have wires surrounding your antenna, and it's just a weak signal, this looks to be a really nice one to get. http://www.newegg.com/Product/Produc...82E16833998041
It's all up to you in the end, and what you can figure out.