What i would do is figure out what sounds you want, pick the software up, and buy a computer based on what the software requires--to me buying the computer first would be like buying a 9v battery and then figuring out what effect pedal to buy afterwards. In any case, I suggest a dual-core processor at least, 2 gb memory and i would check out external hard drives. If you are going with a laptop with windows on it, it is going to burn up a bit of hard drive space, which you might need, as there are softsynths with upwards 2-3 gb of samples per synth, which can fill up quick.
if you want some cheap/free synths, check out kvraudio.com or just do a search for free VST synths on google
