Performance boost for upgrade - small. There is a tier level for upgrade, (could not fined the link).
Cost of upgrade - in your case CPU ($200) and MB ($200 ± 50) and HSF ($30 -> $80)
Approx $500 with reusing all other components
As Dadiggle indicated - go for an SSD.
The biggest Bottleneck on a computer is the HDD and an SSD just blows the door off of an HDD. Program load times are considerably reduced (Also boot time, but that depends on how often you turn on/off or reboot). Should help considerably in programming (compiling and saving to disk).
Cost - Most use a 80 -> 120 Gig SSD for about $250. In your case I'd go with a 240 gig for $450->$550. If you go with the latest/greatest Vertex 3 (SATA III - $560), when you do upgrade ie IB (skipping SB) you can move over.
This option, In My estimate will give you the biggest performance boost per $$$.