mattthemodder wrote:only hard part should be cooling
Got enough room for a low profile 486 heatsink and fan? That'll be 3x better than the original cooling
In many cases, it would not be. The DC uses shielding and sometimes heatpipes to pull the heat away so the fan can do the rest. A 486 heatsink (a really cheap, small, aluminum POS in other words) provides more surface area, but you could no longer properly take advantage of shielding plates or heatpipes. If you're going to replace the heatsink, replace it with something more significant, possibly with a slow (quiet) fan of its own to directly cool it. If you don't have the room for that, a low profile (1U) copper solution may be better.
As for overclocking the DC beyond 240Mhz, you'll start running into stability problems pretty quickly if you go too far. Some SH4s will go further than others. If you really want to push it, you'll have to put the DC in a slightly larger case (for those not already planning to do so) so you can signicantly improve cooling (a Saturn case would be an excellent candidate, actually). You also might want to see if there's a way you can slightly increase voltage to the core.
Onto overclocking other components... nobody has done this yet, AFAIK. But if you do overclock the memory a bit, you're going to actually hurt performance unless the bus is operating at that new speed as well. Heck, even overclocking the SH4 to 240Mhz can actually hurt performance a bit in some situations. Possibly because its running async? Somebody over at dcemu UK was testing this out.