A great source of information is from all of us on here
If you're switching to First-Person Shooters, then you're going to want to upgrade your video card muscle. A Radeon LE (32mb DDR version) would be just fine. You can hack it to enable the missing Hyper-Z and raise the clockspeed back to normal levels, and it's cheaper. They also are supposedly beautiful - especially compared to the nVidia series of cards. ( I have a GeForce3 though, and have never seen a Radeon in action so I can neither verify nor discount this information)
Secondly, let's get into the motherboard decision. Right now, your best choice would be something with the AMD761 chipset. AMD760 northbridge, VIA686B southbridge. The KT266A is an awesome chipset, sure. But there are no motherboards out with it, yet. The nForce will (should) be great, but again - it's not slated for release in the near future. The SiS735 might be something to look for if you're not making your decision right away. Currently there is only 1 board commercially available, and it's not as overclockable as the AMD761 offerings.
ABit and ASUS are great manufacturers. I hear the KG7 from ABit is great. I'm not sure about ASUS' latest offerings. However, my personal preference is with the EPoX boards. I have the 8K7A, but many people swear by the 8KHA (which uses the KT266 - not KT266A (yet) chipset). They're some of the most overclockable boards you can get, and they're stable.
For RAM, you can go to crucial.com . They're great. Cheap RAM, cheap shipping. In addition, they have a script online which shows you what kind of RAM you need given your motherboard. Other choices include Corsair or Mushkin. I've only got experience with the Crucial RAM. All in all, I'm satisfied with the performance, but you do need to give them a little more voltage to get them to perform.
Sorry if I droned on, but there is really no one answer to your question. There are lots of great choices out there today.