I would take a guess of around a 4890ish card. That is if you are ok with a DX10.1 card. There really is not car in that range in teh DX11 market. The 5770 performs under a 4890 and a 5850 is a pretty good leap above.
Hmm, well a new Eco will cost around £60 on ebay, a new GTX260 will cost £130 and a new 4890 will cost £140. So if I get 2 9800GT Eco's, it will come to around £120 and it will only use 50W's of power on full load, per card. So thats around 100W's for both cards on peak load. Looking at the graph below, it shows the 4890 and the GTX260 burning past 250W's on peak.
I do have another question on my mind though. If I was to go Tri-SLI with the 9800GT Eco's, what will it compare to? Don't forget, that the first card will be running PCI-e 2.0, while the other two slots on my motherboard only run on 1.1.