A HD5770 would be about the same as a HD4870 in speed, you can get them for about £100 but I don't think that would be very smart as it's just a sideways step. I think you might need to go 68xx or 69xx to actually get an upgrade. Maybe a GTX460 would be a good one from the Nvidia camp, a good midrange card that's supposedly not too noisy.
I went HD4890 1GB to HD6950 2GB and it's a fair bit faster, around twice as fast maybe but I did need to fit a Thermalright Shaman to it as it's a reference blower fan type heatsink.