If it's for business purposes you probably have to capitalize it, assuming you are running your business returns through your personal taxes, which is a hassle. If it's an unreimbursed business expense you need to make sure you are over the 2% threshold to be able to deduct it. And technically you would need to prorate the purchase for personal and business if you plan to use it for personal stuff at all. If you have a separate personal computer you may be able to claim the full value of the business computer.
I have decided not to bother to claim my next computer as a business expense because the paperwork is silly for a small purchase.