Upgrading Mac ?'s

I have 2 mac computers that are about 3-4 yrs old. They are running 10.5 OS. One is a imac and the other is a mini. I am using the mini headless for music exclusively. Was wondering if i should upgrade it or save for a new one. The imac is my main computer.
Both are only 2gb ram memory. Was wondering if i should purchase extra ram from site like critical.com? It is like $50 for 8gb i believe. I also need to upgrade from leopard to snow leopard then on to lion? Not much of a computer guy,sorry. Any advice is appreciated.
Do they do everything you want them to, and not feel appreciably sluggish or unresponsive? If so, my advice would be to leave well enough alone, and save up for your next computer in a year or so. Otherwise, the RAM upgrade is a good cost-effective solution in the interim.

If they are Intel based Macs and all the software you use still supports upgrades for OSX10.5.8, then I wouldn't do anything other than possibly increase the RAM (4Gb minimum). If you're running into problems with software, then consider updating to a newer OSX. A new computer would be the last resort. You should be able to get another 2 years of good use out of what you have.
I know that Sonos requires the newer OS ( 10.6) to use their upgraded controller. I use that with my desktop. Sadly i would have to go from Leopard to Snow leopard then on to Lion to get caught up to the latest OS.
My Mac mini is ok but it does seem slow at times. I do have to restart it often as i get error codes ( ie -50) whatever that means when running itunes.
I was in your exact situation. I sold my mini on eBay for $350, bought a new one for $600 plus $50 for extra Ram. Glad I did. Cheap upgrade to get up to date. The 2011 mini is reportedly better sounding.
If I were you I'd sell the mini and get a new one. I was in the same situation as you. I sold my mini on ebay for $350 - not bad to get >50% the value of a new one after 4 years. A new one is $600, and the 2011 model is supposed to have better sound, possibly lower jitter. I haven't done any comparisons yet. Anyhow, for $250 invested, you get the new OS, and a machine that is much easier to add RAM to (the 4yo ones are quite difficult). One downside, the newer ones don't come with optical drives - less elegant all-in-one enclosure if you do need an optical drive. But if your are networking with your other machine, you could rip CD's on that one - so you'd be covered in that respect. I say sell it while it has value!