Standard Arduino comes with Atmega328 chip. It isn’t the top notch microcontroller when speaking of number of I/Os or Flash memory. You could go with mega version when real power is needed. But fact is that mega might be too much or simply too expensive. In that case consider Sanguino. It is an Atmega644 based Arduino board assembled on custom board and its cost in parts is about $8.
Atmega644 has more pins and twice Flash memory than Atmega328. The schematic was originally copied from real sanguino so only a soldering has to be done. There is a full support for Sanguino – all you have is to copy some files that makes it available. If you need more power and still need DIP packages Sanguino is right choice.