it depends 😉
SQL Standard can use all of memory which OS can see. So if you have Windws 2003 Ent (x64/x86) or Windows 2003 Std (x64) you can use 32GB ram.
if you are using 32bit os - you should place /PAE switch to the boot.ini
- in this case you SHOULDNT use /3GB or /USERVA switches
And last, but not least - you reeealy should set max server memory option