With 6.3(2) code you should be able use policy NAT to do something like the following (haven't tested this):
access-list 100 permit tcp any any eq 80
access-list 100 permit tcp any any eq 443
nat (inside) 20 access-list 100
global (outside) 20 interface
This (policy NAT) has a higher priority over standard NAT (not including "nat 0"), so this will take precedence over other nat/global pairs you have configured.
See http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_sw/v_63/cmdref/mr.htm#1032129 for details.