Idk, GeForce 4 is a bit OP. Here is my build, which you can follow almost exactly if you like.
Slot 1, Intel SE440BX-2 motherboard. Intel mobos are usually safe on compatibility and capacitor side.
GeForce 2 MX, A GeForce 4 Ti is not TOO overkill, but that's the sorta think you'd be running Half-Life 2 with on a Socket A machine.
Sound Blaster Live! Value. This is controversial. If your DOS games work in DOS in Window, then it's fine. I know for a fact that Duke3D works 100% fine with DOS in Window. Set it to whatever MIDI device you want, load a great soundfont, and boom. For DOOM use ZDoom. Any Windows 98 machine can handle I think the most modern version (If not a very recent one) of ZDoom. Keep in mind, DOOM is VERY CPU intensive. You will LITERALLY get better framed in Half-Life 1 than you will in DOOM. The BIG Snafu with the AWE64 and ANY ISA card is there is no full DMA on ISA. Everything is through the CPU, so there will be greater overhead, and you can almost forget about a decent soundfont as AWE64 memory expansions are impossible. This card can load soundfonts up to 512MB large (If your memory can hold it). The decision is do you REALLY need 120% DOS compatibility, or can you deal with a few games not working? Because you are not giving up nothing with the AWE64.
The CPU is a Pentium 3 450mhz with 512k of cache on a 100Mhz FSB. This is your basic, cheapest P3. It will work fine, don't get me wrong, but there ARE faster cards if you want it. You can go up to around 1Ghz in speed with some boards, which may fit your GPU better, but faster the Pentium 3, higher the cost. Also keep in mind, some people will be dipshits and leave out the cooler for the card. If you want to buy one yourself and apply it, go ahead. Also be wary of OEM parts with no fans on them. I have one, and I have two high static pressure fans directed at them full tilt. It stays PERFECTLY cool, and you could do with regular fans, but keep some airflow on them.
I have 384MB of RAM from 1-800-4-memory (http://www.ebay.com/itm/330591601413). CHECK THE MEMORY YOU GOT IS WHAT YOU ORDERED. This happened to me before when I ordered FPM RAM and I got EDO. They took it back no charge, but BE WARY OF THIS!. But when they DO send you the right part, it works 100% fine. I love these guys because their prices are dirt cheap, and they are near me. Your location may vary. Some people say this board can take twice the RAM, but be aware that Windows 98 has a strange limit of 512MB for RAM. There may be a work around, IDK, look it up yourself, but keep that in mind.
As for a hard drive, I use a 40GB WD400. If you want to keep your sanity intact, DO NOT USE THIS DRIVE. It is the loudest drive I own, but it just happened to be around what I wanted in this case. You can use anything from a good 8-10GB drive, but for larger games, you can go all the way up to around 160GB. Don't break my heart and use an SSD plz, I will cry.
I also suggest a DVD+RW burner and that you get a cheapie spindle of DVD+RWs. I use these when I need to transfer file to and from this machine. Forget USB flash drives, USB 1.1 is slow af. Unless you have a firewire or USB 2 card and drive, DVD+RWs will work both ways perfectly fine. CDBurnerXP 3 for the 98 machine, and whatever you like on your main PC, you can store up to 4.7GB on a disc, and you will never hit the re-write limit as long as you life (If you do, they are cheap to replace). DVD+RWs tend to be better than DVD-RWs, but DVD-RWs have better communicability with some drives (Ignore this if your drive is DVD+/-RW
Your situation may vary. Nothing older than a Pentium 3 for Half-Life 1, it will not run well. My rig can pull HL1 on 100% playable frames max settings with no lagspikes 1280x1024, which is about as good as you can get. SB Live! has a bad rep for DOS, but it tends to work better with DOS in window, and it will run on several games. You COULD try DOSBox, but that is a bit dicey with such a slow CPU.
If you want to push a bit newer, Socket A is a good idea, and it will work with your GeForce 2MX better. ABIT is a good choice for Socket A as their caps on that platform have improved since Slot 1/A. ASUS, MSI, BIOSTAR, and other modern brands are fine too.