Of the two options, the AMD would get the nod as the better performer. Both would be hefty improvements over the current 6000+, but it's possible your AM2 motherboard could be capable of supporting an AM3 CPU. That means you may not have to buy a new motherboard. You'd have to check the manufacturer's CPU support list. (Or provide your motherboard's make/model so someone else can.)
If your board fully supports AMD's Phenom II lineup, you should note that the Phenom II 955 is only $10-15 more than the 840. The 955 very easily overclocks into the 3.4-3.6 GHz range with a simple multiplier change. The stock cooler is typically effective enough for such overclocking as well.
If your current motherboard won't support an AM3 CPU, then you may want to consider saving up for full platform upgrade.
I have played Rift, starting with beta 3 on into release. During beta, I was constantly swapping between 3 and 4 active cores on my AMD Phenom II 720, as well as playing with the frequency (via overclocking). I observed improved performance with 4 cores, and slightly better performance with higher clock speeds. Basically, the more CPU horsepower I threw at the game, the better it played. This was especially true during massive zone events with a couple hundred people zerging the invasions.
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