Well, it depends on what you use the computer for. If it is mostly a gaming PC, then there really is no need for an X2. Dual cores perform no better on games than single cored CPUs do (that is, until multi-core supporting games come out).
If you do a lot of multitasking (ie: burning a cd, while photoshopping) then a dual core might be worthwhile.
A good way to check if it is a good choice for you, check out benchmarks to see how each CPU performs on websites like www.tomshardware.com www.xbitlabs.com www.anandtech.com