I just put together a system a couple weeks ago and researched all the new tech. Here's my recommendations based on only playing AH:
Case:
Wide variety available. My preferences went to Antec, Coolermaster, Lian Li, and Thermaltake
Power Supply:
At least 480W (>500 would be better). Would only get from Antec, Enermax, OCZ, and PC Power & Cooling. If you're on a tight budget, look for a case that includes the power supply.
Processor:
single core AMD
Budget: 3200+ Venice core
All-out: one of the FX line
Mobo:
Budget: nForce4 Ultra (non-SLI)
All-out: nForce4 SLI
EPoX has had a lot of good reviews lately about being a fast, stable, cheap nForce4 board. If you plan to overclock, look into the DFI's. Note the DFI's are also tempermental - they are meant for overclockers to tweak every last bit of speed from the system. You also cannot go wrong with Asus.
Memory:
Budget: Corsair ValueSelect 1GB (2x512) Dual channel Kit.
All-out: Corsair, OCZ, RedlineMushkin 1 or 2GB Dual Channel Kits. There are different types of ram offered by each company. Ram with the lower timings = better = more expensive.
Video Card:
Budget: nVidia 6800GT or Ultra 256MB
All-Out: nVidia 7800GTX
I personally prefer nVidia over ATI for the main reason of nVidia supporting PS3.0. There are no games that support this *yet* but when I plop >$350 on a video card, I plan on keeping it for quite a few years where games may be released that support it. If you have the money, the Mobo, and the desire, go SLI. Just realize that 2 video cards does not equal 2x the perfomance. I don't know if you will even see an improvement in AH.
Hard Drive:
Go SATA. If you have the money, I'd recommend a Western Digital Raptor as the non-budget drive to get. As a budget drive I prefer Seagate and Western Digital due to their reliability and warrenty. Hitachi and Maxtor are faster, but there are more stories of failure from these drives. I wouldn't bother with Raid 0, the performance isn't worth it, but Raid 1 is good for mirroring of data.
Sound Card:
Budget: On-board sound
Non-budget: Creative X-Fi
CD / DVD:
Go with a DVD-RW drive, they are pretty cheap nowadays.
OS:
Go with Windows XP Home or Pro. Don't go 64bit.
This is the system that I ended up putting together. Its a blend between cost and performance with me leaning more towards performance. I'm planning on overclocking the processor and Ram so I paid extra for better, more stable components and cooling. Total cost for everything (including case, cooling, and monitor which are not shown) was around $2200:
AMD 4400+ X2
Antec NeoPower 480W
ThermalTake XP-120 Heatsink
DFI Lanparty nForce4 SLI-DR
2GB OCZ Platinum DDR400 (2x1GB)
BFG nvidia 6800GT 256MB
Seagate 7200.8 250GB HD
Plextor 712SA SATA DVD-RW
Creative X-Fi XtremeGamer SoundCard
-aqualung