USB Wifi devices are always weaker than onboard Wifi (by nature of the more compromised antenna). However, if the onboard one is old, it may only support B/G, so may be benefit in getting N capable, or if the router supports 5Ghz, getting a 5Ghz USB (note - 5Ghz more affected by walls etc).
It may be possible to replace the onboard PCI/PCIe one with a newer type, but double check the form factor and the antenna lead layout.
For USB ones, I have a couple of cheapo Edimax ones here (bought, because they work with Raspberry Pi), and they are "OK".
For a 54Mbps wifi, expect the throughput to be around 20Mbps max - well below superfast broadband speeds offered by fibre broadband packages from Virgin or BT based services. In hostile areas, it will be lower.
If all wifi devices are slower, it may be you are too close to another access point on the same/adjacent frequency (most channels overlap, except 1,6 and 12, and these tend to be the default channels that most routers use, and thus will cause problems if used). Windows tools such as InSSIDer will show what frequency local access points are on, including those with hidden SSIDs.