If you have Arctic Silver on hand, then by all means use it. It is still good stuff.
BUT, if you are about to purchase new TIM, I think that the cool kids are using other stuff these days. Names escape me right now but if you check out the most recent [H] heatsink review, I'm sure you'll find a reccomendation.
OR, there is a cooling section here on the forums where I am sure you could get excellent guidance.
Arctic Silver 5 is still good stuff, but the newer gloop run a couple degrees cooler with no curing time. If you already have it on hand and aren't in a rush, just use it. There isn't a monster difference.
I personally use Tuniq TX-2, and I have some Gelid GC-Extreme on hand that I'll use when I'm not so busy/lazy.
AS 5 is good still but yeah if you can get Gelid Extreme I would go with that one, just use the dot in the middle application no matter which of those you get...it works best.