For an AMD K6-2, I would take a Voodoo 3 2000.
Cheap, silk smooth installation, 3DNow optimised drivers, low CPU dependence, decent performance in games.
It is what I have now. And I think it's great.
Take cheap now, you'll be able to save money for a bigger config later on. Anyway, even if Duron and TBirds are considered cheap, I don't think they'll come to the K6-2 price level before a while.
Even the original Athlon.
And whether you take a K6 or an Athlon, you will most probably have to change the mobo anyway, so...
Maybe the celly would have been better for upgrade.
Main rig: Athlon64 x2 4200+ | Asrock 939N68PV-GLAN | 4x512MB PC3200 | 120GB Seagate | Samsung DVDRW SH-S203D | VX550W PSU | WinXP Home SP3 / Ubuntu 7.10
Headache generator: K6-2 500 | FIC VA503+ | 2x64MB PC133 | Voodoo 3 2000 PCI | SB32 ISA PnP | 15,2MB IBM | Xubuntu 8.04 / DeLiLinux 0.7.2
Lappy: Compaq EVO N115 | Mobile Duron 1GHz | 384MB PC133 | VIA KM133 | Xubuntu 8.04
A big thanks to MS's Vista for making me look into Linux distros!