So I took a chance with a new Asus 560 (non Ti), and I am happy with the result. Basically following the advice above to get a 460 and OC it, but gambling on a 560 instead.
The card is a 1GB Asus GTX560 DirectCUII 850MHz OC. It has improved internal heat dissipation and twin fans. Cost was just over 140 pounds.
So now in (for example) EQ2 with max settings I run at 65 degrees with 40% fan load, rather than 85 degrees with nearly 60% fan load on the Dell card.
I can now play Skyrim on Ultra High settings with just a "gentle" hum rather than a very loud vacuum cleaner-like noise.
Ambient is about 31 degrees.
I would recommend this to anyone else stuck with the awful Dell 560Ti in the XPS, it does make a noticeable difference for me, and while not silent it's more in keeping with what I'd originally hoped for
