Well, firstly, check in bios and make sure you have the timing set to 5-5-5-18 in bios.
Interestingly the label on the sticks is the models I found (IE you said the reaper was C42G, while the label says its C44GK. The label on the gold suggests 1.9v.
OCZ webpage for the Gold, 1.8v with 2.1v OVP
http://www.ocztechnology.com/products/memory/ocz_ddr2_pc2_6400_vista_performance_gold_4gb_dual_channel
OCZ webpage for Reaper 4gig kit, 2.1v default, with 2.15 OVP
http://www.ocztechnology.com/products/memory/ocz_ddr2_pc2_6400_reaper_cl4_4gb_edition
Correcting the timings in bios may help a little, although it could make it worse, make sure you have the pairs matched, normally that means slots 0,2 for the first pair, and slots 1,3 for the second pair, (IE reaper/gold/reaper/gold). If you have the sticks paired as reaper/reaper/gold/gold, then that could cause additional problems.
As 1.9V did appear to give less errors than 1.8 volts, move up to 2.0V, if thats error free, then job done. Otherwise as both types of ram officially support 2.1V, then if your still getting errors increase to 2.1V.
The only other suggestion I can make if that doesnt help, would be to buy another pair, either gold, or reaper, and have all 4 sticks matching, the gold would probably run 4 sticks at
[email protected], while the reaper with its heatsinks would likely run 4-4-4-15 @ 2.1v with 4 sticks. Wouldnt bother replacing all 4 sticks, Overclockers have the 4gig gold kit in stock, so that would give you 4 sticks with the same ratings.
Hopefully though 2.0v or 2.1v with the existing memory will give you error free with timeings of 5-5-5-18 anyway, and still have all sticks within OCZ warrenty.
Dont get too caught up with the need for exactly matching ram, I've got 2 corsair, and 2 ocz sticks in my system and it works fine. The main reason for matching ram, is so that the best rams sticks arnt being "wasted" IE your reaper can do 4-4-4-15, but its going to be stuck at 5-5-5-18 because of the gold.