The accelerated 256-color driver uses 16K bytes of scratch space in video memory, and the hardware cursor also uses 1K (2K on the '6X). Consequently, a 1024x1024 virtual resolution should not be used with a 1Mbyte card.
On older chips, the use of a higher dot clock frequencies has a negative effect on the performance of graphics operations, especially BitBlt, when little video memory bandwidth is left for drawing (the amount is displayed during start-up for 542x/3x/46/6x chips). For the 542x/3x chips, with default MCLK setting (0x1c) and a 32-bit memory interface, performance with a 65 MHz dot clock can be half of that with a dot clock of 25 MHz. So if you are short on memory bandwidth and experience blitting slowness, try using the lowest dot clock that is acceptable; for example, on a 14" or 15" screen 800x600 with high refresh (50 MHz dot clock) is not so bad, with a large virtual screen.
5434-based cards with 2Mbyte of memory do much better at high dot clocks; the DRAM bandwidth is basically double that of the 542x series. The 543x chips also make more efficient use of the available DRAM bandwidth. The same goes for the 544x.