You can do that above, or I can help you assemble some parts from Ebay or the likes. I love making 98 boxes.
Basically what you want to find is a Pentium 3 era system. Typically to me the best for speed and "oldness" would be the 1GHz socket 370 setups that can run PC133. Make sure it has an AGP slot as well on the board.
For graphics you can go with something like a Voodoo 3 to Voodoo 5, or something a little more powerful like a Geforce 2 Ultra or Geforce 3 TI500. My personal 98 box has a 6600GT AGP in it, but my box was also built for speed. If you want all means of nostalgia I suggest a Voodoo card.
Grab yourself a Sound Blaster Live! card for this project if you can. Preferably the Live! 5.1 Platinum if you can although the Live! Value has a nice MSDOS Midi emulator for 98se in software.
The worst part about this project is HDD support. If you have some extra pennies to mess around with I suggest getting a PCI SATA card and getting some WD Raptors. This is the exact card I use and it works on boot without drivers because of it's VIA chipset.
SYBA SD-VIA-1A2S VIA PCI SATA 1.5G / IDE ATA 133 Combo Controller Card / Supports Raid 0, 1, 0+1, JBOD, non-RAID mode - Newegg.com
I use 3 74GB Raptor drives in RAID 0 to make 98se scream. Alternatively if you want to trash a good SSD you can use that too
but I don't recommended that unless you really don't care about the SSD. Otherwise if you don't do the above setup you'll be stuck with IDE drives for the best support and ease of use.
Most Pentium 3 Socket 370 Asus boards are ATX so a standard ATX case and PSU will suffice.
Down to RAM, one stick of 128MB PC133 and one stick of 512MB PC133. You can go to one GB (2 512MB) if you want to use the software I have. Speaking on that, I have a load of official and non-official updates that makes 98se much easier to work with and gives 1GB RAM support as well as a lot of other stability and speed tweaks. If you're serious about it give me a shout and I'll hook you up. The one stick of 128MB is for the install as 98se can be a pain and sometimes BSOD with more than that at first. After my patch you can go to a full GB without problems.