Looking for ram utility.

memtest - you can run in single user mode

you can run 48 or 96 concurrent tests in terminal to test 256 or 512MB chunks for 4-5 loops

you can use Rember which is just memtest with GUI but then you can't run concurrent smaller chunks as far as I could see

ECC is actually a trick to really testing RAM though.

And give AHT Extended Test a run -

http://macperformanceguide.com/Mac-MemoryReliability.html

Intel-based Macs: Using Apple Hardware Test

What part of memtest didn't you understand?

AHT does not look for the type of errors that commonly affect memory, it will spot problems with parity, with daughter card and hardware related.