If you're running XFree86, your server will probe your card and tell you what your highest-available dot clock is.
Otherwise, your highest available dot clock is approximately the monitor's video bandwidth. There's a lot of give here, though --- some monitors can run as much as 30% over their nominal bandwidth. The risks here have to do with exceeding the monitor's rated vertical-sync frequency; we'll discuss them in detail below.
Knowing the bandwidth will enable you to make more intelligent choices between possible configurations. It may affect your display's visual quality (especially sharpness for fine details).
Your monitor's video bandwidth should be included on the manual's spec page. If it's not, look at the monitor's highest rated resolution. As a rule of thumb, here's how to translate these into bandwidth estimates (and thus into rough upper bounds for the dot clock you can use):
640x480 25
800x600 36
1024x768 65
1024x768 interlaced 45
1280x1024 110
1600x1200 185
BTW, there's nothing magic about this table; these numbers are just the lowest dot clocks per resolution in the standard XFree86 Modes database (except for the last, which I interpolated). The bandwidth of your monitor may actually be higher than the minimum needed for its top resolution, so don't be afraid to try a dot clock a few MHz higher.
Also note that bandwidth is seldom an issue for dot clocks under 65MHz or so. With an SVGA card and most hi-res monitors, you can't get anywhere near the limit of your monitor's video bandwidth. The following are examples:
Brand Video Bandwidth
---------- ---------------
NEC 4D 75Mhz
Nano 907a 50Mhz
Nano 9080i 60Mhz
Mitsubishi HL6615 110Mhz
Mitsubishi Diamond Scan 100Mhz
IDEK MF-5117 65Mhz
IOCOMM Thinksync-17 CM-7126 136Mhz
HP D1188A 100Mhz
Philips SC-17AS 110Mhz
Swan SW617 85Mhz
Viewsonic 21PS 185Mhz
Even low-end monitors usually aren't terribly bandwidth-constrained for their
rated resolutions. The NEC Multisync II makes a good example --- it can't
even display 800x600 per its spec. It can only display 800x560. For such low
resolutions you don't need high dot clocks or a lot of bandwidth; probably the
best you can do is 32Mhz or 36Mhz, both of them are still not too far from the
monitor's rated video bandwidth of 30Mhz.
At these two driving frequencies, your screen image may not be as sharp as it should be, but definitely of tolerable quality. Of course it would be nicer if NEC Multisync II had a video bandwidth higher than, say, 36Mhz. But this is not critical for common tasks like text editing, as long as the difference is not so significant as to cause severe image distortion (your eyes would tell you right away if this were so).