Start with what exactly you want to do. For example, if you want to do quilting and also want to do embroidery, my advice is to get one machine for quilting (maybe a Janome 6600) and a machine that will be dedicated to embroidery. That way you can be quilting on a machine while your embroidery is working away (and you just get up once in awhile to change thread colors on the embroidery machine).
I really think the two-machine approach is both more economical and more satisfying.