Just to add an extra thought to Jester's post. A good hand plane would be an even better idea if you also own a router. Before I bought my planer/ thicknesser(Record) I made myself a planing sled from an idea in a routing book. Basically the sled is made up from a small sheet of perspex, size dependant on your needs, mine is about 2'x1'. Along the long sides are two lengths of good straight timber(preferably hardwood) to keep the sled flat. The router is then bolted in the centre of the perspex, with a hole large enough for a good sized flat bottom bit. The only other piece of kit you need are two straight battons, deeper along one side than the thickness of board you want to surface(both battons must be exactly the same. I used two lengths of stainless steel box section). Use double sided tape or a glue gun to fix the board temporarily to your bench, so that it can't move. Now place the battons either side of the work. Your new sled can now span the two battons. Bring the cutter down a bit at a time, each time sliding your sled backwards and forwards, working your way across the board until you have cleaned the whole face, which should now be perfectly flat. Repete on the other face, and you now have two parallel faces. Now a good hand plane comes into its own, as you only need to clean off the marks left by the router to have a perfect board.
It does take a while, and you make a lot of mess, but you will have a job beating this method for the outlay. With edge tools such as planes you really do get what you pay for, and a good plane should last for generations if looked after. I hope you can make some sense of this. If you would like the title of the book let me know and I will look through my collection for you to try and find which book it is in.
Where would we be without the router?