Don't sell your current machine short, either! Having a lot of throat/harp space is really nice but is not NECESSARY for most quilt sizes. I don't roll, I "puddle" so really only about a square foot of quilt at a time is flattened for working and the rest of the quilt is pooled around the machine. Having support IS critical (as someone mentioned) - set up spare tables and/or ironing boards or something around your machine so you're not fighting the quilt's weight as you quilt and it all becomes much easier.
You shouldn't have any trouble doing lap-size quilts on a standard machine. Anything up to & including Queen should be pretty manageable. Even a King is possible, just not as easy. Your lap quilts will be good practice!
I definitely think it's best to save up for what you REALLY want; if you don't you won't be happy and you'll just want to replace your new machine, which is a terrible feeling.