i have an e63 and have never had problems with apps unless they were incompatible. Make sure the app is compatible. Java is java but symbian comes in different versions.
nokia app store is the best and
safest site, and the only one
guarenteed to be virus-free http://store.ovi.mobi/
others u can also try:
http://mobilestore.opera.com/
http://m.en.softonic.com/phones
http://m.getjar.com/
http://wap.mobango.com/
http://phoneky.com/