What "mm" filters to buy for NEX5??

i want to buy uv n polariser filters for my NEX5,
but there are numerous sizes of filters,
as per my knowledge 49mm is comapitable.

But is their any thing of which i aam unaware of?

n also how is "Opteka" filters?,worth buying?
not gonna spend much on filters. max 40-50 dollars.

here are some link, plz tell me your opinions.

Thanks and Regards,
Mahabaleshwar Sadanand.

http://opteka.com/r72ir.aspx

http://opteka.com/fk5.aspx