Ok, I am posting a tutorial on how to make a border on your signature.
1) First create a layer. [Any size]
2) Next select the marquee tool.
3) Put it over your signature.
4) Hold CTRL and press A.
5) Next rasterize it if not rasterized.
6) Then with your cursor right click and press stroke.
7) Choose the color you wish.
8) Press ok.
9) Press CTRL- D.


10) Now you have your border.;)

No offence here but, thats a REALY weak border.

Here's a better border:

Create two new layers on top of everything else.
Make sure your colors are reset, so hit D.
On the first layer, Ctrl-A.
Right click, select stroke, 1 pixel, black, overlay, 50%.
Go to second layer, Ctrl-A.
Right click, select stroke, 3 pixel, white, overlay, 50%.
Create a new document which is 6 less pixels on height and length than your sig. (I.E. on a 450x125 sig, make a 444x119 new doc. Make sure it's transparent background.)
Select all, right click, stroke, 1 px, black, 50%, overlay.
Still selected all, Ctrl-C to copy. Close document, dont save.
Go back to sig, Ctrl-V to paste. Then you can set your white border layer to overlay or soft light.

VOILA!

Well sorry my first guide.. mines an easier way:confused:

Easiest border of all!!--->Make new layer
Crtl A[select all]
->Edit>Stroke

VIOLA!!!