There is no major differences right now between the 128 and 256 in the 9600 series. I think 256 on a 9600 is overkill.
I know you asked for "the cheapest" 9600 pro, but if you can afford it, you may want to go with an XT version. These are faster than the pro, and not very much more money.
Also, I would stick with Asus, ATI or Gigabyte. Sapphire is good too, but personally, I like the first three I listed.
ps. Some cards come with a Half Life 2 coupon, you may want to check that out.
CPU - P4 2.4C, 800FSB W/HT - CONSERVATIVELY OC\'ED
MOBO - GIGABYTE GA-8KNXP 2.0 875P CHIPSET/DUAL BIOS VER. FE
RAM - 1GB (2X512) OCZ PC3200 REV 2, 400MHZ 2-3-3 6
VID CARD - ASUS RADEON 9600XT 128MB W/VIVO
FLOPPY - SAMSUNG 1.44M
HDD - MAXTOR 120GB 7200RPM ATA133 EIDE W/8MB BUFFER
DVD - LG GDR8162B 16X IDE DVD ROM
PSU - ENERMAX EG465P-VE FC 460 WATT
VANTEC VP4-C7040 AEROFLOW HS/FAN
5 X VANTEC STEALTH 80MM CASE FANS