i tried art rage, didn't like it. the canvas feature was really weird.
SAI is a really, really nice program. it's like a mix between photoshop and open canvas, not much like painter. you can download the japanese 1.1 version for free here, i believe: http://www.systemax.jp/en/sai/
and a 30 day free trial of the english version here, which has more brushes and is slightly better: http://sai.detstwo.com/sai/
and i don't know where you can buy it, but it's only like $50.
open canvas 1.1 is a very good painting program, and it's free. you can download it here: http://wistinga.online.fr/opencanvas/
and you can usually find programs like painter x and later versions of OC on e-bay for very cheap, especially if they only have the CD and no upgrades or manual.