I tried Clockworkmod method:
Step by Step Guide
1.Backup your SD card data to PC before start partitioning.
2.Power on your phone and donít let go power button.
3.DFT MAGLDR Bootloader will be loaded.
4.Press call button to load 1. Boot AD SD.
5.ClockworkMod Recovery will be loaded then.
6.Use volume down button to go down to Ďadvancedí.
7.Then press call button.
8.Select ĎPartition SD Cardí.
9.Select the ext size that you want (128M, 256M, 512M, 1024M, 2048M or 4096M).
10.Select 0M as the Swap Size.
11.Copy back your data to SD card.
My mobile is Samsung i9000, and loaded with cwm to enter into recovery mode,
After step 10, it said finished(looks like it works), but I reboot into system and found the sdcard is the same capacity before repartition, and put it back to computer, no ext3 partition shown, put into pe, it's the same one partition here.