Nah, there shouldn't be any incompatibilities, but unless you want to run any game from late 1998 and 1999, I'd stay with 64mb because there really is no reason for more.
Plus, some motherboard chipsets, like the i430vx do not cache more than 64mb RAM, which means that 128mb is actually going to be slower!

He may just load a ramdrive in the top 64 MB if he needs the speed of cached ram in DOS.
For some games it is also a good idea to just leave the lower 16 MB free.
Wing Commander 4 shows for example this problem, if its own memory manager just gets free RAM beyond 16 MB. It can not use it.
(f.e. if you loaded a 16 MB smartdrive buffer)

So maybe just fill everything beyond 16 MB with a ramdrive (on top of memory), load smartdrive (or corresponding caching driver) with ~2 MB. This gives a quite compatible environment for DOS on machines with a lot of memory equipped.
Hmm does some caching driver supports using memory on top?

I remember some time ago, when I've been trying to play the "Aladdin" game. It wouldn't play on my pentium and pentium 3 pcs. The first had 128MB RAM and the second 512MB. It would display the message "xms allocation error....". I remember back in 1995 I would play that game fine on my pentium 100 with 8MB ram. I was surprised when the game actually started when I ran it within windows 98, but couldn't get my soundcard to work. Then I found out that if I left up to 32MB ram on the board it works fine running at dos. So for me, the best is to have up to 64MB when using dos, and perhaps with the help of ramdrive reduce it to 32MB. Also with 64MB and a fast mmx Pentium, windows 98 se are running ok.

Thanks for the answers!!!

I was using this machine with 64MB and like elianda said, I did get into some trouble , IIRC Realms of Arkania 2 needed 32MB or less to actually work. For that purpose I am using a ramdisk program to eat up either xms or ems and then unload it with a simple .bat file grinning face

As for the speed in 64/128MB configuration, I did made memory benchmarks using Sandra 2001 in win98se . My mobo is MVP3 based btw.
Speeds where the same (just a couple mb/s difference depending on the module(s) used, IIRC ~78mb/s and ~110mb/s using the fpu) but what it did made a difference was enabling bank interleave (with a value of 2 or 4) that becnhed ~87mb/s and ~135 with the fpu (or is it mmx read?) . Again using 1x64 or 2x64MB dimms made no real difference in the benchmark.

edit:

@Markk : yep win98se is running OK with 64MB until you open an IE6 window to download a patch or a driver... then is is "welcome to slow swap city" ... 128MB really did made a difference. And I am not using a pimped system with a gazillion apps running, only drivers installed, IE6sp1 and the unofficial SP. ....now if only win98se wasn't such a crashfest on my system dizzy face I am always getting those recoverable messages about explorer or ie...

IIRC Realms of Arkania 2 needed 32MB or less to actually work.

sidenote: if run thru Win98, it actually works fine with 512mb memory... beaming face with smiling eyes
(and yes, that includes full sound etc, no need for pure dos)

I think you should keep the 128MB, and install a 64MB+ disk cache in the upper half.

You can do that with UIDE, which is an all-in-one disk cache, cd cache and cd-rom device driver. It can load itself above 32 or 64MB and uses only about 200 bytes of conventional ram IIRC.

edit:

here's the link: http://johnson.tmfc.net/dos/driver.html

Interesting thread, i put 256 mg in my celeron-500 dos machine, it boots to dos and win95. I figured it would not bother the system much and is useful to run utility live-cds such as hiren's that rely alot on ram when using linux or windows based sessions, i did a low level format on my 20gb prior to installing w95 and mini xp + the program need 256mb to work

From what i read here, Ill have to keep an eye on my ram if i run into some problems with dos games, maybe ill just downgrade it when im done setting everything up on that system.

jwt27 thanks for the suggestion, I'll grab the prog and see how it goes beaming face with smiling eyes