i use a gigabyte gmm 6800. get it online for 700.
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else if you are serious about photoshop, get a graphics tablet like wacom bamboo.
these babies are not cheap (the decent ones cost upwards of 8k, and the really good ones start at 15k), but they are just unbeatable. no input device is better than them at photoshop/graphics work. its almost like drawing on a piece of paper. all the pros use it, and with it, you can create stuff which is impossible with a mouse (like pressure drawing, fade brush, etc).
your current mouse (as you said yourself) is pathetic, so go for a new mouse, at least.
also, get a proper mousepad, it'll help a lot. coz due to uneven surface, the scanner may skip and mouse may behave unexpectedly.
tl;dr get a mouse and mousepad, if you can afford a graphics tablet, good for you.