for virtual instruments on a pc, you only need midi out on your keyboard. i originally got a small format keyboard and after using it for a couple of weeks i regretted not getting something with more keys. originally i was going to use it only for drum stuff but after playing with some of the synth and sampler plugins at kvraudio, i was infected with the vsti bug. i got reason 3 shortly after that. man it's alot of fun. i'm a guitar player but i have fun playing around with all the different instruments and synths. it's like the ultimate musicians toy.
i would skip the small format keyboards and go for something as big as you can afford. buy used if you have to. knobs, sliders, and buttons come in handy for controlling instrument parameters and your recording software controls. if money is tight, find a cheapo used casio or something with midi out to use for now and save up for something bigger. it's a pain in the ass having to use the octave up and down buttons when you just want to player a couple of higher ocatave notes. it kind of kills the flow.