I'm not sure how much ram you have available, but it cant hurt to try it out. I use it all the time and just consider Krita and Sai additions to my arsenal so to speak. Basically, since I only have 3 gigs of RAM and Windows 7 likes to eat 1.5 gb ish just to run the OS i'm limited on memory too. I usually am okay if I make sure other programs aren't running. If I have my browser, I only have one or two tabs open. What eats up memory quickly is having a lot of layers. I usually flatten / merge a lot of layers together periodically and try to keep the files small.
In case you're wondering about hardrive space, my Krita program folder is only 270mbs so that's pretty small imo. As long as I keep the brush sizes down it seems to work fine even on my old computer.
It's a little weird to get used to, but it's not to bad. Most graphics programs are sorta modeled off Photoshop, so like Sai for instance, has a lot of the same features and hotkeys. Both Sai and Krita can import PSD files too, it's just not all the layer effects will work (unless you flatten the layer) so it's best to save those for last. Basic layer modes like 'multiply' work in both programs it's just the fancy stuff like drop shadow stuff like that can be a little iffy. You get used to it pretty quick and it doesn't seem to be an issue for me really.
Of course it's up to you. Photoshop is a pretty amazing program in it's own right.