I regard Avid as the natural competition for FCP in the professional editing world, but whether another package is appropriate for you depends upon what precisely you’re intending to use it for.
If you’re not at what might be termed the top end of the editing market it might be worth considering alternatives, not just Premiere but also Grass Valley’s Edius and Sony’s Vegas.
How long a computer will last is hard to say. Disk drives have physical movements with contact between components, so there’s a chance for wear to shorten their lifetime, but for components that have no moving parts the sensitivity is to physical shock/vibration and excessive temperature variations. So if a computer (or other electronic device) never gets too hot for you to put your hand on it and is kept in a nice cool and stationary place it’s likely to last a lot longer than one that is moved about and never has the cooling fans cleaned. I have seen computers last for more than 10 years (with the occasional hard drive/power supply replacement), it’s often the desire to have the latest software and/or higher processing speed that drives their replacement rather than having them fail.
So I can’t say whether it’s best to keep your G5s going or not, but if they’re doing the job you require them to do efficiently at the moment the question arises of what is the risk of them failing to you? If the inconvenience of having one go down when you’re working would be serious it might be wise to replace them while you have the opportunity.