CX is slightly better than CXM. With any decent psu the chances of DOA are slim, the better the quality, the lower the chances. There are better options at Amazon but they cost more, Thermaltake Toughpower Grand, Antec Gold, Corsair RM, Seasonic Focus, Corsair TXM etc, but they are in the $85-$99 range. I'd love to see you go with better quality, better protections, better warranty, but it's not my money to spend.
If it wasn't for the spikes I'd be happier saying jump on the RM 550w, but that's a gamble, your pc will be running 150w heavy gaming, add in 350w worth of spikes and you could easily be seeing 500+w semi continual usage. The RM is easily built to handle that kind of load and abuse, it's just a question of how much do you put faith in your gpu. It's older, used, not brand new, does it have slightly higher power needs to maintain clocks, the 1100MHz OC versions hit 364w spikes. In just that one game tested. Or it could be you never see over 300w and be perfectly good topping out at @ 450w, well within the RM 550w ability.
For me, I'd bite the bullet and get a better unit, or wait to see if I could get better locally, on sale, combo cpu/mobo etc to open up options in budget, save up the extra $20 somehow.