So my bike is currently rocking old Stan's Crest rims, which I believe have 21mm internal width. As I'm using 2.3/2.4" tyres I'm looking at them, and looking at all the reviews of wider rims, and thinking should I swap rims at some point? Will I actually notice a difference in adding a few mm to the width?
Stan's reckon that for a 2.25-2.5" tyre I'd want their new Arch rim at 26mm. Any opinion on this?
It depends on the riding you do really. There's no point buying Arch rims unless you need the extra strength as I don't really think a few extra mm tyre contact would be worth the weight penalty.
I'd say it also depends how firm you run your tires, I was running 20ish psi on similar rims and the tires would roll a bit in corners, now with 31mm inner width rims the roll has gone and it feels much better cornering. If you run say 30psi, likely less benefit.
Running 2.4"s on 30 mm XM 481's, a smidge heavier than the rims they replaced but the way they've improved the ride has been more than worth it.
The benefits of wider rims have been covered in depth here so I won't go into them again but I'd at least advise you give it a go.
If you do and don't get on with them then a least there'll be a nice queue of people who want to buy them off you.