Reseting Jailbroken 3g

[psm_uk]

Hi, my first post in the forum so here goes, I just bought an iphone 4 and will be selling my 3g problem is I want to wipe it but it is jailbroken. The last time I tried to reset it when jailbroken it all went a bit wrong and locked up etc. It is on os 4.2.1 I dont know what jailbreak it is apart from it has a snowflake on it when starting up, can it be wiped of contacts etc without to much trouble? if so simple answers please, many thanks for any replies