CHDK will definitely not let you shoot faster than the the built in continuous rate, and will be substantially slower using the simple intervalometer included with chdk.
http://www.usa.canon.com/cusa/consumer/products/cameras/digital_cameras/powershot_sx260_hs#Specifications is a bit confusing, but it looks like your camera can do between 2.4 and 0.8 FPS in continuous mode depending on settings.
If you don't need a steady frame rate, you could just use continuous mode, using press() to hold the shutter down for the desired time or number of shots.
The standard intervalometer will try to do a full preshoot (auto focus, auto exposure, etc) before each shot, so it will be substantially slower than continuous mode. Using the kind of script discussed in
http://chdk.setepontos.com/index.php?topic=1444.15 (which holds down halfshoot and repeatedly clicks shoot) should be substantially faster. Of course, this will not adjust focus and exposure between shots.
On my d10, the first (full preshoot) method tops out at about 3.1 seconds/frame, while the second manages about 1.6. I forced 1/256th exposure for this, if you are shooting in low light the actual exposure time can be significant. I also used MF, which reduces the time required for the first method.
On some cameras there is a low light mode which is 1/2 resolution but much faster than normal shooting. Using CHDK, you can force the ISO to a lower value which regains some quality. Using this method on my d10, the first case is reduced to 2 sec/frame.
Normal (non low light/iso3200 etc) reduced resolution modes do not seem to shoot faster than full resolution, and in some cases are slower. High ISO settings that aren't a special reduced res mode can be slower too.
SD card speed probably isn't very significant if unless you are using something really terrible, but having a fast one won't hurt. My test above were done using a very slow card.
Forum user lapser has been doing some interesting work on faster intervalometer capabilities, discussed in these threads
http://chdk.setepontos.com/index.php?topic=8997.0http://chdk.setepontos.com/index.php?topic=9172.0This is his own custom code which is not currently included in CHDK, but he might have build you can try.
