A flash drive would be fine, but not ideal for a large music collection...1GB flash drives are the most common, and a 4GB flash drive can be pretty expensive. Instead, I'd suggest an external hard drive. For the price of a 4GB flash drive you can get a 60GB+ external hard drive that plugs into your USB port. While not quite as portable (unless it's a mini drive it won't fit in your pocket), it's a much better value.
Hope this helps,
My husband needs to take a great deal of music with him in the most portable way possible and play it back on his laptop. Would a new laptop with a bigger hd be a better choice than a USB flash drive? The reason we were thinking the USB instead of a new laptop was the ease of transferring the music files from the computer he uses in his recording studio.