Are there any issues using the cards newer than the 4350 in regards of low dot clocks? I seem to recall from my days using soft-15khz that the 4350 was the last card produced to achieve low dot clocks...??
Good question. The way I understood it, the dot clock limit is based on the actual design of the card and the components used, not the specific chip, though could be wrong.
I wonder if there's any way to test the dot clock and start some sort of list of cards (by exact model number) with low dot clocks.
According to the experience with Linux (and the patches done by bitbytebit), the dotclock lower limit seems to be software related. There's probably no physical limitation of the hardware that prevents it from getting lower dotclocks. But drivers limit achievable dotclocks between some upper and lower limits they get from the videocard's rom (not always).
A dotclock is a frequency value. When you request a given dotclock to the driver, it needs to program the PLL dividers, in order to convert the videocard's master clock frequency (i.e. 400 MHz) into the frequency (dotclock) we are requesting. It's an arithmetic problem. Drivers contain specific algorithms to achieve that, which depend on the chipset. So some chipsets may work better with some values than others. The problem here is that not all achievable dotclocks are stable, so picking the right algorithm is critical as this is a common source of troubles for driver developers it seems.
Now, for older ATI cards I've tested (R 9250, R X300), even if they're BIOS sets a limit for their lower dotclocks (which required a specific patch in GroovyArcade), that value seems to be happily ignored by Windows drivers, which admit nearly any imaginable dotclock for these cards.
The problems started with later models of the X family, and for the whole HD 2000 and HD 3000 families. For those cards, Windows drivers seem to use a dotclock's lower limit of around 7 MHz. That makes them not the best choice for emulation, as they will refuse to work with resolutions lower than 384x or 400x, unless you set huge porches to artificially increase the dotclocks required. Fortunately we can still use these cards by creating modes with double width, so Mame will scale the frame and the result will be perfect. But anyway, it's better to pick one card that can natively do really low resolutions, as there are more emulators other than MAME that may not be so smart.
Fortunately, HD 4000 family can, again, do any low dotclock, as it was the case of older cards. We don't have patched drivers for anything above HD 4000, and probably won't, so one card of this family is the best choice if you wan't to have a full working 15 KHz card while still have a relatively modern (or not so ancient at least) card.
Probably that 7 MHz limit of the other cards could be unlocked by patching the Catalyst video drivers, but that could (or could not) be a collosal task, and after all you can get a more modern card from the HD 4000 for a few dollars.
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