After working for Viking for 9 months, there are three major things about I'd consider important about buying a machine. I ended up buying a Platinum 750Q after it had been discontinued for about 50% off. It's an awesomely sweet machine and I highly recommend the Viking Platinum line.
1) You get what you pay for. There is a direct correlation between quality and price. I'd recommend getting the best machine you can afford to get.
2) You should never pay full retail price for a sewing machine. Discontinued or floor models are great if you can get them. At Viking, we worked mainly on commission. Even if there wasn't a sale currently on, if a customer pushed and prodded for a better price, we could usually get permission from the manager to give them a sale price just so that they wouldn't walk away without buying a machine. At the very least, we could throw in a few extra feet or other accessories. One woman who was buying a high end machine (a couple thousand dollar purchase) ended up getting a 10% discount as well as $300 box of embroidery feet and hoops.
2b) If you can't afford a machine, I'd recommend getting a job at the sewing machine store! Not only do you get to drool over your machine of choice every single day while working towards buying it, you can also get an awesome (-40% at Viking) employee discount.
3) Make sure that your machine comes with a warranty longer than 3 months and that it includes labor. Even the nicest and awesomest machines can have issues at the beginning of their lifecycle. One machine I took out of the box to set up as a floor model had been damaged in shipping and had to be sent back for repair immediately. Repairing your machine is expensive and the technicial is (hopefully) a highly trained professional. Standard repair costs at my store was $80 (labor) + parts.