The Speedtouch Frog can cause problems on some hardware because it drains too much power from the USB port. Some (older) VIA chipsets saw this as a hardware fault and would shutdown the computer... great
Also, with USB modems when you turn off the computer, your connection drops. If you have any intention of hosting your own site then this is not a good idea. When your connection drops, you lose your IP address. When you reconnect you get a new one... and it is not likely to be the same one (certainly Freeserve do not offer static ip allocation. I believe Tiscali do, but you have to specifically ask for it). If you use an ethernet modem/router that is just left on then your connection is never dropped so you can use your system to host.
However, if you intend on putting a 'spare box' to good use as a firewall, then a USB modem that is supported by the firewalling OS (Smoothwall, IPCop, etc, etc) should be fine as you are likely to leave the firewall machine on anyway, so it'd be just like a modem/router.
Oh, and if you are thinking of getting an ethernet modem router, ebuyer have some that are relatively cheap. I was going to get one myself until I found out that although my exchange is adsl enabled, I am just a little bit too far into the wild for it to work. I think the actual terms the BT engineer used were "snowball's chance in hell".
Good luck.
