RouterOS USB Boot
The latest version of RouterOS (starting from v3.0rc5) has support for booting from USB devices added. This new feature allows you to test how your PC would work as a router. Also it could be a good option to run the RouterOS from USB with the newest version to test out how well it is performing. It means that you can install the RouterOS on the USB flash/drive.
To install the RouterOS on the USB you can use CD-Install or Netinstall from the latest versions of the RouterOS (starting form v3.0rc5). Also it is possible to attach that USB drive to a Windows computer and copy the RouterOS installation files using Netinstall to the USB drive and when booting this USB drive it would install the RouterOS on it.
Media Capabilities with Motherboards and Laptops
Since there are lot of USB media types, below are notes for most of that which have been tested.
USB hard drive
USB Flash Stick
On Gigabyte GA-8I945P-G-RH motherboard that are using (945 chipset) you need to go in the BIOS and specify to boot the from the USB-HDD. If you have another hard drive installed in the same PC then you should also specify the boot priority. For the Gigabyte motherboard in the BIOS it gives two options regular harddrive that you have installed and Bootable add-in drives. You should specify the Bootable add-in cards. After the first reboot if it didn't boot from the usb drive, you need to reboot once more and go into the same section and there you will see a new boot option - the usb flash drive name.
USB Card Readers
Not only USB flash/drives can be uses, also USB card readers can be used. In these USB Card readers you can plug any media (CF, SD, xD, etc) and use that as a bootable device to run RouterOS. But there could be problems with these Card readers as sometimes it is not recognized as a bootable device in BIOS - such thing happened on the Intel DG965WH motherboard (965 chipset), but it worked on Gigabyte GA-8I945P-G-RH motherboard (945 chipset). It could be because of the motherboard BIOS and that shouldn't affect the regular USB flash/drives.
The booting support for the USB drives is the part of the BIOS, so if your motherboards BIOS doesn't allow to boot from that drive you might try to upgrade the BIOS of the motherboard.