You will be making some compromises sonically by placing your speakers on a shelf inside of a entertainment center. In essence, you will have a box inside of a box and speaker companies really do not design their speakers to work optimally in this type of situation ... though in reality, it is probably a more common set-up than they might imagine. Typical audiophile speaker concerns, such as ... imaging; soundstage; separation; ability to disappear ... are difficult to come by in this type of set-up.
With this in mind, there are some things that you can do to make the best of this type of set-up. You may want to consider acoustic suspension(sealed box) or front ported speakers ... back ported speakers are pretty much out, unless you want to plug the ports and then, what's the point. You can also experiment with placement ... whether to place the speakers standing right side up or on their sides. With side placement, you can also decide whether you get better separation and dispersion with woofer in/tweeter out [o O] [O o] or vice versa speaker placement [O o] [o O] . If possible, try to place the speakers on a shelf that is close to ear level height (or even slightly higher) when you would be sitting down. Finally, you may wish to go without a center channel speaker and use your dvd player's ability to create a simulated surround sound effect ... this can be just as effective and pleasing, as having a center channel speaker.
You will still be able to get good sound from this type of set-up, but you really do not have to spend an outrageous amount of money ... because you do not have to chase all the typical audiophile concerns. Take a look at NHT SB2 ($400 list); Wharfedale Diamond 8.1 ($200 list); PSB Image Series; and Monitor Audio Bronze Series.
My 2 channel HT system features speakers (Acoustic Research 302) inside of an entertainment center and it sounds just fine, given all the above listed constraints.
Good Luck, Rich