Those averages should have you wearing...what you normally race in. 51 is ideal. not too cold to hurt performance, but cool enough to keep you from over heating. Tri shorts, a tri top, hat maybe. I'd add arm warmers. A shell vest, maybe. Some folks like a long sleeve Dry Fit or Under Armor shirt under their singlet in really cool conditions. Personally, in too cool conditions, I like a light pair of gloves. You'll warm up quite quickly once you are on the bike. and will be even warmer still once you get to the run.
The problem with gear that can keep you dry ( Gore-Tex for example) is that it is unable to get rid of the moisture you produce (sweat) at a fast enough rate. You get soaked form the inside out. I'm a sweater too and have found that over the years it's just better not to worry about a shell.
Everyone is different, so if you can, experiment. A few years ago I did a local oly (Columbia) in horrible conditions. 45-50 degrees and driving rain. There was a high DNF rate that day but I was ok with the exception of my feet which didn't warm up till mile three.

That's a great idea... But then again, my tri top does have these cool things sewn in the back called "pockets". I use them to keep stuff that I'm not using, but don't want to throw away.... ;)

Ya, not too much to worry about until doing HIM or IM distances - at IMWI '06 it 'warmed up' to almost 50 that day with constant rain and wind. Made it through with gloves and arm warmers - tho many DNF'd with hypothermia. There are so many options out there that it is difficult to choose. I am also a sweat bucket guy, so I will be wet regardless of the gear. Best thing is to pick up some gear and then ride/run out in the crappy weather during training. Then you can see what your tolerances are and get the appropriate gear. Generally layering is best. Also important to find out if the gear causes chafing etc. during training and not a race.