Get a decent processor. Try to get 32 Gb RAM if possible (or 16 Gb). Currently a 32 Gb desktop kit is fairly cheap (~$150).
Also booting (and running simulations) off a solid state drive is preferred.
Make sure you get a Quadro video card. They are more expensive but you can experience stability issues with gaming cards. I would get a Quadro 2000 at minimum (although a Quadro 4000 would be nice with Photoworks - though they are not cheap).
Are you sure you need a new PC? What are your current specifications? What problems do you currently see? As this will help us spec.
Check out the Admin forum for lots of advice on PC builds. Note that most of SolidWorks only uses one processor thread, whereas FlowSim will use many, so a lower clock-speed / higher number of cores may be beneficial in this environment.
my current specs.
Xeon post processor E5620 2.4ghz
2x ATI fireproV4800
500GB hd 7500rpm SATA