jEnus, Dell 33xx are Delta Networks, Broadcom chipset. Dell 26xx are probably somebody else, definitely Marvell chipset. Don't know about the other lines.
Dell shops around, and passes the savings on to you. Each major Dell product variation is likely to come from a whole different bidding cycle among the possible vendors, and so could possibly be different every time. This also has the VERY important consequence that the quality of one Dell networking product (good or bad) does not imply anything about that of a different product, so qualify them carefully!
I've had good experiences with the 33xx switches and 52xx switches. The 26xx switches, did some research, wasn't even interested in bothering to test them.