I am getting back into mountain biking after a few years riding a desk at work.  I am considering upgrading my RST 461 forks.  I will spend up to £160 but I want to be certain that for the (mostly) leisure xc riding I will do I will see the performance benefits.

 Are Rockshocks Dart 3s worth the effort?

Unquestionably yes.

For £160 you can get a surprisingly good fork these days, either Marzocchi or Rockshox have decent forks at this price.  I bought my g/f a set of Dart 3s to replace the RST Gilas on her Hardrock and I'm quite surprised as to how good they are.  

I think the benefits will be huge

My mate has a pair of those I think it would be fair to say that they are heavier than a rigid fork but little gain in control or shock absorbtion

£130 I think will get you some Marzocchi Mx pro (maybe 2007) you'll need a new headset (well may be not need but benefit from). Maybe a shock pump?