Actually, *any* version of Photoshop including age old 6.0, 7.0, CS1 are fine.
Even better, the LE versions of Photoshop are perfectly suitable as well (which can be found for a few dollars or for free if you scourge your family/friends cd's - often bundled with digital cameras, scanners etc).
If you like my signatures, keep in mind that they are 99% handwork. It could be done with something like Paintshop Pro, Coreldraw, even Indesign or Illustrator or similar vector based package.
In the end, anything except paint will do