I suggest getting the "latest", as this hardware will just last longer, games will install and play faster.
A good MoBo, having official support for W98SE and XP, is this:
http://www.asrock.com/mb/Intel/775i65G%20R3.0/
Can be bought i.e. new or used on amazon.
Choose the fastest CPU that is supported without hassle, and makes the machine snappy on XP:
Intel Pentium Dual - Core E5700 (3GHz)
Can i.e. be bought new and used on amazon.
2 x 256 = 512MB DDR-400
Can i.e. be bought new and used on ... guess... amazon.
Any GeForce 6600 or 6800 AGP with EXACT 256 MB Video Ram.
Can i.e. be bought new and used on ... you do the math... just joking... ebay.
Any Soundblaster Audigy (1 or 2) PCI, for best sound experience
Can i.e. be bought used on...