I had also chosen the E8400 and GIGABYTE GA-EP45-DS3R until I found I was ending up over budget. The EP45-DS3R would be the first thing I'd upgrade if you want to spend the rebates before you get them
So I cut back a bit on the CPU and MB to keep the 4870 in the mix. The E7200 will OC to 3.0Ghz on the stock Intel HSF but to be honest in most games the 3.0Ghz won't make as much difference as the ~20-25% difference the 4870 has over the 4850.
pqi TURBO and WINTEC AMPX RAM are a good value for performance RAM. AMPX was in this THG review: DDR2-800 Goes Gold
The E7200 is 2.53Ghz which works out to be a 266.5Mhz FSB (1066 quad pumped) times the 9.5 CPU multiplier.
If you overclock the E7200 to 333Mhz FSB (1333 quad pumped) times the 9.5 CPU multiplier you'd have a 3.16Ghz CPU (roughly E8500 equivalent)
This would be about the max OC you could expect with the stock Intel C2D HSF. A good E7200 can reach this speed without extra vCore increase.
Taking the OC to 400Mhz FSB (1600 quad pumped) times the 9.5 CPU multiplier you'd have a 3.8Ghz CPU which is a bit more than 50% OC. E7200 review