Well doing the letter in a vector-based programme is always a good idea. You could then use them to create bigger pictures without any loss of quallity. Most of the nice effects are only available in pixel proggrammes, though...

If you need the letters just for a sig, Photoshop will get the job done, if you are going for printing quallity you should always leave the letters in vector format... Cause when the document is rasterized for printing the rasterization of vector objects is done with a step of 0.1 or even 0.01 pixels

Best vector software in my oppinion - Adobe Illustrator (since they added transparency in version 9... or was it 10 )

Corel Draw is also worth a look but it's somewhat heavier cause it tries to be both Illustrator and Photoshop at the same time

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