You'll need Odin to flash and you'll have to look here:
xda-developers - View Single Post - [Firmwares]Stock I9000 and I9000M Firmwares collection [Latest: XFJV5, XWJVN, UGKG3]

To find the Froyo build for you particular carrier/location.

 

Who are you getting your service from?

I haven't used Odin myself yet, so can't really provide any input on that aspect of it.

Instead of downgrading to Froyo, you might want to try upgrading to 2.3.4 instead.

 

Go to your phone dialer and dial *#1234#

This will tell you your current CSC value, then you can find the correct build to install.

 

Hi I just upgraded my phone to 2.3.5 using Odin for the very first time its a bit nerve-wracking the first time. I got all of the info needed from News - www.SamFirmware.com the site is horrible to navigate but all the info is there. Also the downloads are hosted externally and it only lets you download files 1 at a time every 30 mins unless you pay a premium fee.

Basically you download and install the driver.

Download the firmware package which includes usually 4 files (the pit file which is formatting instructions, the csc, modem and PDA files a quick look at the names and you should be able to figure out what is what)

A Note on the firmware the name of each file is important as it describes the language and countries that the firmware supports (a full list is available on the site).

Your firmware is i9000XXJVO - i9000 is your handset xx English + some of europe and jv again english + some europe and the last letter is the version. I forget which is region and which is language but as long as any firmware you flash is xxjv then it will be correct.

There is a walkthrough pdf on that site also although the first 3-4 pages are info that is not required power setting etc repair notes.

Put you phone into download mode (read the pdf)

Run Odin - set the required settings and choose the files to flash

Connect the phone via usb (make sure usb debugging is turned on first)

Click start and wait, once complete your phone will reboot load some more info reboot again and you will be up and running.

Remeber to back all your contacts and data etc up before you do this as it will be reset.

Everything you need is here Login - www.SamFirmware.com

2.3.3 Hotfile.com: One click file hosting: GT_I9000_XXJVK_XXJVK_OXAJVK.rar

2.3.4 Hotfile.com: One click file hosting: I9000XXJVR SWC.rar

2.3.5 Hotfile.com: One click file hosting: I9000XXJVS.rar

Older versions are listed on the site and there are some alternate region/language codes that are valid but I would try and stick to xxjv to be safe.

I use spbshell 3d and in 2.3.5 it's noticably quicker than 2.3.3

If you get stuck just shout

UTR