Are you looking to offload your files for storage or are you wishing to use the files directly from the external drive?
Let us know
In that case you will be better off using a Firewire drive rather than USB as it will allow for faster transfer of data.
Probably something like te Maxtor one touch turbo series will do what you want http://www.maxstore.com/searchresults.asp?search_id=3&dept_id=58 theXXXXXis only $350.00 and is capable of data transfer up to 800mbps so you shuold see almost no difference betweenyour internal and external drive access times.
Let us know if you need more info.