Go with the E6300 for sure. Why would you pay around $100 or more for a single-core processor that isn't going to give you all that much better performance at stock speeds in (single-threaded) games, nevermind a better experience overall?
Go back and read part 1 and 2 of the Core2 Duo articles here on Anandtech if you want to see some benches.
The E6300 was 10-20% behind a FX-62 at the worst at stock speeds, so figure for a FX-55 it would be less of a difference. And overclocked, the E6300 will beat even the FX-62.
Save some money and get the E6300 and you can put the savings towards something else in your rig.
And for the record, over the past couple of years I've had a P4 540, A64 3200, 3800 and 4000, which I just swapped out for a E6400 so I don't really favor one side. The dying chipset fan on my Asus A8N-E was a sign from god it was time to move to Core2
