ATmega8 - 8K flash,1K of RAM
ATmega168 - 16K flash, 1K of RAM
ATmega328 - 32K flash, 2K of RAM
So, anyone knows when new chip is released and what spec it has?
Planing to order few 328 chips soon, but if new upgraded version comes in near future then I can wait a little. 
In that package, dunno of any that would be much better.
There is of course the 644 in 40 pin DIP...
At the rate Atmel is shipping 328s, and at the "typical" time between when they announce a new chip and actually ship it, and given that they haven't announced any new 28pin DIP chips for a while, I wouldn't expect any improvements in the chips that you can put in an Standard Arduino board within the near future...
There are a bunch in the 644 family larger package
164A/164PA/324A/324PA/644A/644PA/1284/1284P with
16/32/64/128K Bytes In-System Programmable Flash
with more IO and dual hardware USARTs.
Or did you intend to stay with the smaller package of the 168/328 based boards?
I was playing around with making a small card to mount on top of the 40 pin package here
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1289437587/15
since the leaded surface mount version of the 644 doesn't appear to be currently purchasable.
Build up a little PCB, glue to top of the package, bring 11 wires down to the leads directly underneath some pads. microsystem on a chip 
I know others have done similar by installing the parts right on the dip directly, I was going for something that was easier to repeat. Tried selecting decent sized/spaced pads to make hand soldering easier (on me) vs those little microchip sized parts.
Or make a sandwich, have a small breakout board underneath the dip also. That's also shown there.