You can custom build a really nice rig for around 1200 tbh. I do it for clients all the time. Of the three you listed, if I HAD to use one of them, I would opt for Alien simply for the fact that they have been in the gaming rig market for awhile.
If you have the time and are willing to put in a little work and learn a little as you go, you can save a ton of cash and build yourself a really nice rig for a lot less money and you don't get scripted support when you call for it.
If you choose this option, you can go to either
NewEgg or
Pricewatch for ideas.
In fact, here is one I whipped up just for the hell of it to give you an idea with a ballpark price.
Motherboard - ASUS M2N-E SLi AM2 nForce 500 SATA2 DDR2
CPU - AMD Athlon 64 x2 S.AM2 6000+ Dual Core
Heatsink - HIGH-SPEED FAN WITH THERMAL COMPOUND
RAM - 2gb Pc2 5300 667mhz Ddr2 Kit 2x1gb Chips
HDD - 250GB SATA150 7200RPM HARD DRIVE
Vid Card - nVidia GeForce 8600GT 256MB PCI-e SLI HD
CD #1 - CD-R/RW - INERNAL CD BURNER DRIVE IDE
CD #2 - DVD Burner DVD-RW Drive House Brand
Sound Card - CREATIVE SOUND BLASTER LIVE 7.1 24-BIT
Floppy - MITSUMI 3.5" BLACK FLOPPY DISK DRIVE
NIC - 3COM 10/100 PCI NCI NETWORK 3C905 CARDS
Case - SuperTower ATX Multi-Fan LCD Window Case
Power Supply - 600Watt Dual Core Certified Power Supply
Case Cooling - 2x Case Cooling Fan 80mm
OS - MS WINDOWS XP PROFESSIONAL EDITION
Speakers - Midiland 5.1 Multi-Channel 6-Piece Set
This setup, while basic and not really super planned out as to best of whatnot for price, would run you $1219.00 give or take. And from that you can swap out items, mix and match, order certain things from one place and certain from another.