Ok, so your rooted, are you using apps 2 sd?
If you want a custom rom, you should start by taking a look at one of leshaks kitchens. Look here ...
Http://forum.samdroid.net/f55/samdroidmod-kitchen-v-2-all-one-samsung-spica-i5700-en-1901/
The general steps you need are,
1) sign up to the forum
2) select the mods you want (cook your rom)
3) download the rom by being logged in and clicking in the cook button
4) copy the new xxxxxupdate.zip to your sd card root (ie e:\xxxxxupdate.zip) do this via usb plugging your phone to pc, or by removing sd card and plugging that into card reader on pc.
5) power off phone
6) boot phone into recovery mode (vol down, make call, end call buttons)
7) Select install update.zip
Phone should install, and reboot, reboot might take longer than normal, but no longer than 5 mins
Job done.