You could do what I did to my P233 box: use it for DOS gaming. Just slap Win98 on it and it should work quite well. Mine does DOS gaming decently even with the tiny amount of RAM that it has (I desperately need to get more). I don't think Gens would run full speed on it though, my mom's P200 (non MMX) ran Sonic and Knuckles at 35 FPS in a 320x240 window with no sound.
Thoughtless: That would probably defeat the purpose of the computer. He's trying to see what he can do with it for little to no cost. Buying new components would cost too much money.
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Main PC:
Asus P6T Deluxe, Intel Core i7 920 C0 @ 3.7 GHz (195x19, 1.3v core, 1.84v PLL, 1.3625v uncore) w/Thermalright Ultra 120 Extreme, 3x2 GB G Skill PC3-12800 @ 780 MHz, 7-7-7-21-1T, 1.66v; ATi Radeon HD 4870 1 GB @ 790/1000 w/Zalman VF-1000, 300 GB WD VelociRaptor, 4x500 GB WD RE2 (RAID 10), 3ware 9650SE-4LPML RAID controller, SB X-Fi Titanium, Seasonic M12D 750w, CM Stacker 810, Windows 7 Pro 64-bit, Dell 2007WFP
XBL/PSN/WoW: Thordrune