Don't get the K7V, that's a SlotA board. All new AMD chips are SocketA.
Right now the best boards are the Asus A7V, ABit KT7 and my personal favourite is the MSi K7T Pro2 (or Pro2A, it just has ATA100 instead of 66 not a big diff)
The chipset to get right now is the Via KT133, it supports PC100 or PC133 SDRAM and 100Mhz DDR (200Mhz) Front Side Bus Athlons/Durons (which is almost all of them, there are 3 1Ghz+ Athlons that use 133Mhz DDR (266Mhz) Front Side bus but they aren't big yet)
There is also the Via KT133A which is just around the corner that supports PC100 or PC133 SDRAM and the 100Mhz DDR or 133Mhz DDR (200 or 266Mhz) FSB Athlons I just mentioned (almost identical to the KT133, just adds the 133Mhz FSB Athlon support)
The 3rd chipset (also not available quite yet) is the AMD760 which support 100/133Mhz DDR FSB Athlons and also supports DDR SDRAM PC1600 or PC2100.
The 3 boards I mentioned above are all KT133 boards, as the KT133A and AMD760 aren't available yet.
As for which chip to get, well the Duron is I'd say on average 90% of the performance of an equally clocked TBird Athlon. Get the Athlon if you can afford it. If not get a Duron.
(Not sure if you know this or not, but the TBird is just an AMD codename, officially the TBird is called Athlon, just like Coppermine and P3)