Personally I would just upgrade your video card "If you can I know some motherboards that are store brand and was Duel Core you didn't have a PCIE slot"
I run rift on a mix of medium/high settings my computer spec is only slightly better then your's.
My Computer Spec Is
Motherboard: Foxconn A74GA
Operating System: Windows 7 Ultimate 64-bit
Processor: AMD Athlon(TM) II X2 B22 Processor 2.80ghz
RAM: 6GB DDR3 PC3-10600
Video Card: XFX Core Edition 1GB DDR5
Resolution: 1280 x 720.
Upgrade your Video Card and I would personally have 6GB of Ram but 4GB should be fine I personally think its your video card holding you back.
Edit: Also is that resolution your running at native to your monitor because if its higher then that will give you fps problems. My monitor is a odd ball and its native resolution is 1280 x 720.
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I haven't checked it out on my new one yet sooner or later ill get around to it just downloading 30gig's can be boring and I barely got any HD space left atm.