K100d: should I stay or should I go?

I've been a Nikon user in the past (after using Konica for a long time).

My dad got me a K100d when he got a K110d for himself, and so I switched to Pentax.

Lately I've been wanting to get better lenses - still using the kit lens, though I invested in the nice 1.4 50mm, which it turns out is too big to do much with due to the lens factor.

So, I was talking to coworkers about getting maybe a Pentax or Sigma 12-50mm 2.8 or similar, and how I fear spending more on the lens than on the body, and they said I should ditch the Pentax and go Canon or Nikon before I waste any more money.

I know pentax is a good brand, but I don't see the plethora of lenses that are supposed to be out there. If I switch, I'm looking at around $700 for either a T1i or D5000, and then whatever lenses from there on out.

Thoughts, opinions, lens suggestions?