I know someone who has a DS at Two Waters and is very happy. The school also consistently has excellent SATs results but has only a 1 class intake I believe (30 students) so you will need to live pretty close.
With regards secondary school, if you live more towards the Apsley side of Hemel Hempstead then you may be in the catchment for The Hemel Hempstead School. As there are no partially selective schools in Hemel Hempstead then it is based purely on Sibling and distance from the school. It is important to find out from Herts Schools which is your nearest secondary school (you can call them to find out). The most oversubscribed school in Hemel Hempstead is The Hemel Hempstead school (in 2010 had 818 applicants for 180 places). If this is not designated your 'nearest' school then you are unlikely to be offered a place. In both 2009 and 2010 places were only allocated down to Rule 4 (i.e. distance where it is your nearest school). If it is your nearest school then as long as you live within 5K then you are probably OK. In 2009 the further place allocated under Rule 4 was 5.3K and in 2010 it was 5.5K.
If you are going to move, I would focus on where you are catchment wise for secondary school rather than primary.
I hope this helps a little and you can never plan too early!