I would suggest to get rid of rs232 chip and put there an rtc chip with a backup battery instead. The usage of a Sdcard (for e.g. logging purposes) require a valid timestamp mostly. This is my experience as I run 1284puino for a year now (rtc pcf8563 and cr1220 hardwired on the board).
Rs232 - most gps modules are 5v or 3v3 ttl, so no need for a rs232 hw driver, just bare 3 pins, I think. I would put 10k pullup on each Rx pin as well.
Reset button - I would rather go with it..
Also - I would go with 3V3 Vcc, no need for SDcard buffer and 3v3 reg then. I would go with microsd socket only.. Add maybe a header for an external accu (with a charging circutry or a trickle charge..).
Also mind the situation you have a SDcard plugged in and you do ICSP programming.. The crystal value - you may use any provided you will not use micros and do recompile the bootloader. The delay() used in 0022 is based on micros(??) so I am using my own based on millis (as it was in past). P.
PS:

two new gadgets in the town:
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http://www.digilentinc.com/Products/Detail.cfm?NavPath=2,719,896&Prod=CHIPKIT-UNO32