I had a 9mm Sigma for awhile, yeah the trigger was weird but it was a good gun. I'm with the masses here - I'd leave it alone. Just shoot it while you save and then buy a little nicer gun.
BUT.... do not rush out and buy a Glock as soon as you are ready to upgrade. Yes it's a better gun than what you've got but it may not be "the best for you" in that price range. 3 or 4 years ago I decided I wanted a 9mm, so I read reviews an decided I'd get a Glock or an XD. I then went and shot a Glock as well as an XD - then I bought an XD. I have since shot MANY more Glocks and a few more XD's and my opinion is unchanged, I prefer the XD in every way.
I highly recommend shooting every pistol at your disposal and then picking the one that fits your needs.