What computer/monitor is best for a beginner photo editor?

I am a beginner photographer and have just begun learning how to photo edit (using Adobe Photoshop). I have a Canon Rebel. Most of my photos are of landscapes from travel or candid shots from events I host. I will be buying a new computer and monitor and am looking for advice. Which of the following options would be best for someone like me? I added some of my own comments. Put more simply, I suppose this is an iMac vs PC question. Thank you.

Old iMac: $1099. Con is you get about half the memory and hard drive space as a new Dell, HP, and iMac.http://www.bestbuy.com/site/Apple%AE+-+21.5%22+iMac%AE+-+4GB+Memory+-+500GB+Hard+Drive/2658077.p?id=1218343209949&skuId=2658077
http://www.dell.com/us/p/inspiron-one-23-2330-aio/pd.aspx?~ck=mnNew Dell: 1100 - 1300. Depends on options. Good reviews but some complaints about monitor.
http://www.amazon.com/HP-TouchSmart-520-1050-Desktop-Computer/dp/B005M1AF3O/ref=sr_1_4?s=pc&ie=UTF8&qid=1353019403&sr=1-4Best Selling/Highest Rated HP on Amazon: $1000