If it was unlocked by the original American carrier, it can be used on any GSM carrier with their SIM card, though not all iPhone features may be supported.
If you unlocked it by an unauthorized method (hacking, jailbreaking), there's no way to know if it's still unlocked and no support can be given here.
Virgin Mobile USA is a Sprint subsidary using the Sprint CDMA cellular network. CDMA is NOT SIM based. So I suspect you need to actually get a Virgin Mobile provided cell phone with the Virgin Mobile USA firmware factory installed direct from Virgin Mobile USA.
Virgin Mobile USA is offering iPhone 4 (8GB for $350) and iPhone 4s (16GB for $450) phones. You have to buy the iPhone up front at full cost, but then you own the phone, and there is No Contract. The least expensive plan is $35/month 300 talk minutes, unlimited text and 3G data (with data throttled after 2.5GB/month). Provide a credit card for automatic charging and there is a $5 discount.
You could sell your current iPhone and use the sale proceeds to offset the cost of a Virgin Mobile iPhone.
Note: I got my own Virgin Mobile USA iPhone 4s last month, so I've been through the pricing information recently.