Apple's ( AAPL) much-ballyhooed "halo effect" is spreading to SanDisk ( SNDK). Apple's inclusion of NAND flash in its latest music player -- the already wildly popular iPod nano -- has underscored the potential of the memory technology. And that's helped SanDisk, the inventor of flash storage cards, the largest supplier of flash data storage card products, and the holder of several lucrative NAND flash memory patents. SanDisk shares have hit new 52-week intraday highs for the past four days. Shares reached as high as $53.37 Wednesday before closing down 34 cents, or 0.7%, at $52. The company's stock has more than doubled in the past three months alone, with a climb of about 31% since Apple unveiled its slick nano on Sept. 7. Shares of SanDisk were recently down 15 cents, or 0.3%, at $51.85 on news of two analysts downgrading the company based largely on valuation. Indeed, the impressive run-up of shares means that investors who were once fighting a loud bear argument about NAND flash becoming a commoditized market are now countering charges that SanDisk is turning into a pricey stock. "We struggled for some time on whether or not to trim our holdings," Mark Mowrey, a senior analyst with Al Frank Asset Management, recently wrote in the company's "Prudent Speculator" newsletter. On one hand, he said, the shares are getting expensive after doubling over three months. But on the other, the shares aren't that expensive, at 27 times 2006 earnings estimates and less than 23 times estimates when excluding the more than $8 in cash and equivalents on the company's balance sheet, he said. By comparison, rival Lexar Media ( LEXR), which hasn't posted a profit since March 2004, is trading at more than 125 times 2006 earnings of 6 cents a share. But another competitor, M-Systems Flash Disk Pioneers ( FLSH), is trading lower than SanDisk at 21 times forward earnings and is preferred by some analysts because its top- and bottom-line growth is expected to surpass SanDisk's, although it's off of a lower base. Mowrey's conclusion: Continue holding the shares for further gains until they reach $56 -- a price target the company had nudged up to $49 just a week earlier. Either way, it's a nice profit considering the firm's Buckingham Portfolio picked up SanDisk shares for $24.29 in May.