I to would go with the K6-2+ 450 from Tiger for $34.99.
An example of the improvement he will see would be better than what I saw going from a K6-2 @ 392Mhz to a K6-III @ 488MHz, both non-plus versions.
With K6-2@392,
Nascar 4 scored a whopping 5fps, with everything turned down to lowest quality, highest performance, using either a 3dfx V3 3K, or a GF2 MX.
Switching to the K6-III@448, and same tests score around 24fps.
I would say that IS a noticable speed increase.
The advantage of using the K6-2+ is it's built using .18. All of us Duron owners know how great .18 overclocks, and this is in the same class. Most of these 450's easily run at 600MHz, so the test results posted above with a K6-III @ 448, would be much higher still.
As to the Duron upgrade. MONEY.
Most K6 owners have AT case, AT motherboard, K6 CPU, 2+ year old video cards, and probably a few ISA cards, as well as PC66 or PC100 memory. Even without buying memory or a video card, the cheapest you could upgrade to Duron components is around $300 shipped (including ISA cards, as 1 ISA slot if you are lucky with socketA). Without good memory, and a great video card, performance is not all that great. For example, my Duron 600@935 using 3DMark 2000, scored around 3000 with the Voodoo3 card, 5400 with a GF2 MX card, and 7300 with a GF2 GTS card. There are many K6-2+ users getting 3200 3dmarks.
I should be receiving a K6-2 450+ to upgrade the wifes K6-III 400 in about another day or 2, and will post how well it performs.