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a seagate 7 series? I wouldnt use it, i managed to find it on newegg, it appears to have been new in '04 when they were using 40GB platters
http://www.newegg.com/product/product.aspx?Item=N82E168...
It will be incredibly slow by todays standards there have been a great many advancements since the 7 series, it may not even be functional anymore if it went unused for many years, the magnetic plates do degrade.

You should be able to use it, but it is old, it is slow, and depending on how it's been treated, it may not be as reliable as a modern drive.

Thank you for your reply ,but my question remains,can I use it with my already exsisting hard drive as extra ram? I really dont mind if its slow.

As extra RAM? No - RAM is a different kind of (much faster) memory. You can use it as extra storage space though.

Personally I would lean more toward adding a USB DRIVE of 4gig or more and make that my swap drive to add RAM