I'm assuming you're running Workstation on your PC rather than vSphere/ESXi, so have a host OS like Windows 7 on it too.
As a general rule VMware advises not running more vCPUs in your VMs than you need, i.e. start each with 1 or 2 and see.
The latest hyper-threading gives you more performance but not as much as extra cores, e.g. my lab server uses this http://ark.intel.com/products/65719/Intel-Core-i7-3770-Processor-8M-Cache-up-to-3_90-GHz which has 4 cores or 8 threads including hyper-threading. However your i5 processor does not have hyperthreading so you only have 4 cores/threads.
Therefore if you are running a single VM my advice would not to go above 3 vCPU to start with, leaving one core for Windows. Of course if you're running applications on your desktop (even web browsers can use a fair amount of CPU with dynamic page updates etc) you may need to allocate fewer. Definitely there's no benefit in you having more than 4 vCPUs on a single running VM - the hypervisor would just processing spend time switching the cores for the VM when there's no underlying extra resource.
If that's not enough processing capacity for your VM it might be worth upgrading the processor to a core i7 like I've got (<$300).
Don't forget to look carefully at memory too - if it's a database you're running make sure you don't overprovision it, e.g. if you've got 8GB RAM and Windows 7 64 bit I wouldn't recommend more than 6GB RAM for the VM (or 4GB if you're running some Windows apps). If you don't have one, an SSD wouldn't do any harm either 
Good luck!