making swap partition during install

[mushis@mail.lafayette.edu]

Dear people,

I ran into a small problem while going through the debian installer for 3.0r1. 
During the hard disk partitioning phase ...how do you createa swap 
partition...cos when I try use the command "C [startfreeblock] /dev/sdaX", 
select 512Mb then type Linux swap as the type of partition I get an error  
about bad size??! Later on when I try to make this swap partition useable 
the installer doesn't find it so I always end up choosing "Do Without Swap 
partition". ..is the lack of a swap partition really deterimental to 
performance? I have a beige g3-400, with 512Mb Ram and 18Gig SCSI 
disk.

Thanx