The main problem with gaming laptops is the graphics card which is usually limited compared to its desktop sibling.

Processor speed, RAM and hard drive wise you can relatively match with a laptop.

Also, consider the fact that it's harder to find good OEM laptops that you can assemble whatever you want inside of it like you do in desktops.

Your budget is a bit rough, but I know Dell has a few offerings (some has a higher price tag, but it's lower than Alienware).

For example, Dell XPS M1710, though it starts at $2,299 (I would generally check the XPS series since they marked as gaming laptops).

In general I would take the following items into account:

• The best graphics card you can get for the laptop you are looking at. This is a big issue. Make sure you get an ATI or Nvidia ones and not the embedded cards which are quite bad at games
• A 7200 RPM hard drive (in the desktop you can get a SATA that runs at 10000 which is quite a performance boost which currently you will never get on a laptop). 
  
• 2Gb of RAM
• At least an Intel Core 2 Duo. Do mind that due to the cache sizes on the Core 2 Duo processors for laptops they are a bit more expensive than desktops, so because of the price you might need to compromise on it.

In general, the compromise points are in the graphics card and the CPU.

If you can avoid the need for a gaming laptop, I would go with a normal machine and perhaps make sure it has one of the LAN game party cases like the ones Thermaltake makes if you still wish to carry it around.