There are a couple rules of thumb for testing crank potential. The farmer method, my stand-by, is to disable the ignition and crank the engine, the battery should maintain 9.6V while cranking.
The other method is to use a fixed carbon-pile to put the battery under the load, same test standard, 9.6V and it's a keeper.
The first place to start testing a battery is by checking the alternator, a simple 14.4 V , is your target for maintaining batteries, drop a loop off the stator's windings and you wont see that, anything less in normal circumstances is not going to put the battery to bed properly.
As far as replacements, my $0.02 is with Optima gell-cells , as it seems that there can be problems with just about any battery, the gell-cells will hold up a lot better and seem more reliable.