1-) buddy i would strongly suggest that you try not to root or install a custom rom on the Galaxy S . i9000 is one of those android phones which you can brick easily ! .
I would suggest having it done from someone who knows what is he doing ! . If you can visit karachi , i`ll gladly help you out in rooting and installing custom roms 
2-) What firmware you are on ? i am on XXJVO . It gives me around 36 hours of battery time with a heavy usage . Install a custom Rom like Galnet MIUI / Darky`s Rom , which will give you a good battery time . But will lack some features as compared to the stock Rom .
I was on Galnet MIUI a few days back , beautiful ROM i must say . Gave me 2.5 days of a battery timings , but it lacked camera features + FM radio
3-) Use handcent SMS , highly customizable and good on memory as well .