It kinda depends on your system memory, because under win98, you reduce the available memory by the buffer amount. So if you have 64meg, and allocate a 4096 buffer, you have 64-4(buffer)-16(windows system memory)= 44meg memory for games/apps to play with.
Anyway, this one only works under win98/winme. Find out the IRQ of your network card under the 'device manager'. Mine is IRQ11. Under the system.ini file in your windows directory, find the heading marked [386enh], and at the bottom of the entries, add the line 'IRQ11=4096'. Replace 11 with whatever your irq happens to be. You can allocate any combination of numbers, but I usually keep it to round megs. Here's what mine looks like:
[386Enh]
ebios=*ebios
woafont=dosapp.fon
mouse=*vmouse, msmouse.vxd
device=*dynapage
device=*vcd
device=*vpd
device=*int13
keyboard=*vkd
display=*vdd,*vflatd
DMABufferSize=64
PagingDrive=C:
MinPagingFileSize=131072
MaxPagingFileSize=256000
IRQ11=4096
If you have a dialup modem, you're not going to notice the speed, so don't bother. If you have dsl/cable, 4096 will help on noticably on downloads, and ever so slightly on uploads. Cable can't handle (at least mine can't) much more than 800k/s download, so 4 meg is plenty. If you have a t1 or t3, a 8192 buffer will crank in a noticable difference.
If you care to do a little further tweaking/reading, browse over to <A HREF="http://www.speedguide.net" target="_new"> Your DSL/Cable on Speed</A>