As above I invested in a cokin "P" holder and some hitech ND filters just last week, cheaper way to filter shots when you have one lens with a 52mm thread, another with a 58mm and a zoom with a 67mm.
Imagine how expensive it would be if you bought screw-ins for the different sizes!!!!!
P.S avoid cokin filters if you can as they are not that good. Hitech is the way to go if your on a budget.