The best boosters are the mast head type, putting a booster behind the TV normally means your boosting a weak dirty signal into a strong dirty signal.
anyhow, if it has to be a set top booster then make sure it's one with adjustable gain on it, you will often find the results are better with the gain turned down a bit from full (full gain often overdrives the input stage on the tuner resulting in a worse picture)
Try the 'Maxview VSB 1C. Has a gain up to 20 dB which you can manually control There are higher dB ones available,but i think this is quite good,and all i need!