have you overclocked your cpu at all? If I was you, I would worry more about the cpu bottlenecking the rest of the system. Getting gtx560 ti sli won't be wise since your cpu is likely to bottleneck it even at 4ghz.
However, seeing as guild wars 2 won't be very hardware intense, your gtx260 should be more than capable of handling it maxed out. If you really want to stay at the safe side, then get a single gtx560 ti. Your cpu will still bottleneck it but not as much as gtx560 sli.
IMHO, you should consider getting a new motherboard along with a newer generation quad core i5/i7 cpu. So you might wanna save up your money for that. Intel z77 motherboard, i5 2500k/3550k, 8gig g.skill 1600 ram, gtx 560 ti will cost you around $600-700. Why not stick to the gtx260 for the time being and save up a little more for that.