You can get 2x1GB Patriot @ 2-3-2-5 on newegg for like $130 right now (after rebate). I got this RAM a couple of weeks ago, and I hit 220mhz with those timings NP, and with the 5:4 divider and looser timings I hit 275mhz =2.75ghz on my 3700+.
And I do notice a difference, but not a huge one. It's only in FEAR that it boosts my frame rates significantly, in the average range of 10fps.
Is that 2.6ghz opteron you mention an overclocked opteron?
And my reading experience is the OCZ Plat DDR500 or the mushkin REDLINE DDR500 are the best available for OC'ing. Twice the price of the Patriot though, and its supposed to get up to ddr500 on some setups.