Experiences with buying online?

Hi all,
I'm a newbie to this. Recently got engaged with a temp ring and my fiance has set me loose to pick out a ring I like (within a budget of £1500). As far as I can make out the best value seems to be online - is this really the case? I have found it difficult to get upfront information re 4Cs from jewellers on high street, being just fobbed off with "oh all our stones are high quality" blah blah blah. I like the way everything seems quite upfront re 4Cs online.

As I don't wear rings normally, I'm not fussed about carat size and would prefer something small and neat. When I tried on a few rings I found that I prefered the look of three stone rings, or else very small solitaires with side-stones. I've decided to go with a platinum band which is chewing up a huge chunk of my budget, but I reckon that I can always upgrade the diamond at a later date when we're a bit more flush.

I found this on diamonds-usa.com:-
Platinum ring set with two sidestones totalling .32 carats and G VS2 very good cut to Ideal-Cut, with round centre stone as follows
Color: I
Clarity: SI1
Cut: Ideal Cut
Weight: 0.38 ct
Measurements: 4.71-4.75*2.81
Depth: 59.4%
Table Diameter: 60.9%
Crown Height: 13.5%
Pavilion Depth: 42.5%
Description: AGS-"2" Ideal-Cut

It says that it is certified by Diamonds-USA, which I am concerned about. Are they reputable for certifications? It seems a bit strange to me that they certify their own product. How much is duty when buying from the US?

There is also a ring at http://www.serendipitydiamonds.co.uk...asp/sku=R3D010 that I have spotted, but doesn't mention cut anywhere that I can see, and I don't think they certify stones under .3 so prob no certification either. Is that really a big deal?

What do people think of these? Does anyone have any other website they can recommend? Any advice at all would be great!