Im a newbie but from what I can gather there are two types of machines, thermoblock and boiler. The thermoblock is the common Sunbeam or Breville type and while it does the job they are considered to be of much lesser quality, mainly beacuse they cannot control the temperature of the water as it goes onto the coffee which produces poor tasting coffee.
The boilers are the next level up and typically you go from $100-200 to $400 - $1000, everyone agrees that the Silvia is the ultimate home machine and these are around $750.
A great article is at:
On eBay you can get a lot of refurbished Gaggias if you budget is limited.
Really it depends on your budget, if you let me know how much you want to spend then that makes it easier, also does it haved to be new??