Yes, but not with only 1GB of RAM. Yosemite needs at least 2GB, but for good performance you should upgrade to at least 4GB. Check OWC's website to buy RAM: http://eshop.macsales.com. You can Google around for other vendors.
Macs' operating system is OS X, not iOS. You cannot upgrade to Yosemite (10.10) directly from Leopard (10.5.8). You will first need Snow Leopard: http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard. The disk comes with 10.6.3. Once you install it, go to > Software Update, and update to 10.6.8. Then you can go straight to Yosemite. Updating to 10.6.8 will give you access to the Mac App Store which is needed to download Yosemite.