Yes, your low memory is most likely a RAM issue. Logic 9.0 does not do 64bit. You would have to upgrade to 9.1.8 (I don't think there's a lesser option. I am running 9.1.6 and it's all working fine). If you still have only 4 gig of RAM, I'm not sure that 64 bit will make much difference. Maybe you are in a situation where every bit helps.
Regarding running the external hard drive, you have specified what kind of drive, but not how you are going to access it, which I think is the major issue. Are you planning to run it USB? This is a bad option for audio.
I upgraded the internal 5200 rpm 120 gig drive in my 2007 Macbook Pro to a 500 gig 7200 rom drive. I was offered a 750 gig and should have gone for it, but oh well. But there have been no problems. it runs much better than the original disk, and is definately fast enough for audio.
What you could do then would be to keep your current projects on the internal drive, and back them up to USB drives, and pull them back to the internal drive if you need to revisit them later. That would work much better than trying to run audio from a USB 2.0 drive.
Re upgarding RAM, this would always be a good idea, if you could do it. Especially if you want to have other apps open at the same time (Mail, Safari, Melodyne, etc). 8 gig you would certainly see a difference.
But by the time you upgrade RAM in an older machine, and upgrade the drive, you may be part of the way towards a new machine.... And if so, you would want Thunderbolt external drives, wouldn't you?