Some good reading about keycaps:
http://deskthority.net/wiki/KeycapEssentially, keycaps have 3 important factors: top shape, contour and height. The top shape can be spherical (so a ball would rest nicely on the cap), cylindrical (like most standard keycaps, curved in one direction) or flat (like Apple laptop "chiclet" keys). Some keycap sets have different angle for each row (sculptured or contoured) and some use the same angle for all rows. Different keycap "profiles" can also be different heights.
There are slight differences in the actual angles of the tops of the keycaps, too, which results in a different "feel" between profiles.
Ducky keyboards use what is called "OEM" profile keycaps and quite standard layouts (1.25x modifier keys, standard size space bar). Here are the 4 most common ones:
OEM profile is a cylindrical, contoured, fairly high profile.
DCS is silimar, but lower height.
Cherry is also similar and lower than OEM, but a little different to DCS (perhaps lower than DCS, too?). This profile gets a lot of love here on GH, but can be rare to find sets in it.
DSA is spherical, unsculptured, low profile.
Signature Plastics (also known here as SP) don't make OEM profile, the closest is DCS. AFAIK all the pimpmykeyboard sets are from SP. These are the profiles made by SP:
http://keycapsdirect.com/key-caps.phpSets sold as Filco, Ducky or Keycool are mostly OEM profile. It's nice to match profile when adding novelty keys, so they don't look too out-of-place unless they have their own shape anyway.
Then there are materials:
ABS is "normal". Wears fairly quickly and can become shiny, can be used easily to make "double shot" keycaps using two different colours of plastic for each key so the legends are actually different color plastic. Legends can be printed or laser engraved or etched or doubleshot (doubleshot is best).
PBT is slightly rougher, "drier" texture plastic (a little like unglazed ceramic, IMHO). More dense so keycaps are heavier, but difficult to make doubleshots since the material warps easily when cooling. Often dye-sublimated legends (which last longer than printed ones) and sometimes laser engraved or etched. Most people like this material the most for keycaps.
POM is more "teflon-like". Slicker surface, more dense even than PBT. Usually printed or laser engraved legends. I like POM the most since it is the densest and I like the slick feel of them.
Often the caps which come with cheaper boards are thin ABS. Thick, doubleshot ABS are a big improvement, thick PBT or POM usually even more (but it's a personal thing).
I believe your board came with "thick" PBT caps, for reference. Ivan is currently running a group buy for Cherry profile thick PBT black sets (blank and black dye-sublimated) which I think would go nicely with the yellow case (although the backlight wouldn't come through the caps, but would be visible as a general backlighting around the edges):
http://geekhack.org/index.php?topic=50935.msg1117903#msg1117903