A better card than a TNT2 M64 would show an advantage over the onboard.
As far as I can tell, the onboard is a SIS630/730 - and later drivers may be available from
http://WWW.SIS.COM than those at Pc-chips.
The 630 appears to be the "Intel" version, and the 730 the AMD, but otherwise similar - with a Sis300 graphics core.
The frame buffer allocation is just like the ram size of a card, but is deducted from system RAM - there is probably also an AGP aperture setting, which determines an additional availability of system RAM for communication with the card or onboard.
The main drawback of onboard, is that when the RAM is accessed by graphics - for screen refresh or accelerator functions, the CPU will have to wait - the effect on processor perfomance similar to running with slower RAM - so if the board suppports running the RAM at 133 and the Duron CPU at 100 (200FSB) - use that option unless you find it unstable.
Better still, give it a worthwhile AGP card, or pair a PCI with AGP or onboard if you fancy dual display - dual AGP is not an option!
http://www.xbitlabs.com/mainboards/sis630.html
Unless you have the FBC (Frame buffer cache) option plugged into the AGP slot, it should be DOWN on a TNT2 - but that's a FULL TNT2, not the memory-crippled TNT2 M64.