asbshyam :
I'm planning on adding a HD4850 or GTS 250(occasional gaming). Is it still necessary to go for a 790x board from 780G??? Will there be any change in the HT freq or anything else???
Raven has listed some good combos. Add this to the mix:
PhII 940 / Biostar 790gx: $281
http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.165048
An HD 4830 would be fine for occasional gaming - the Sapphire is $75AR and pulls less watts at idle than the Powercolor.
The 780g has the sb700 and a single PCIe x16 slot - the 790gx has sb750 (improved performance) and a second PCIe x16 slot which may be used in a Crossfire gaming config OR for a second GPU for parallel processing in multimedia apps (as the tech improves). There are no real combo deals with the PhII 940/780g that make them superior to the 790gx combos (if you are purchasing in the USA).
Your RAMs are important in multitasking - look at a 64-bit OS if possible.
The HT should not impact your decision
unless you consider an AM3 microprocessor like the PhII 720BE. The stock HT on AM3 CPUs is higher than the stock HT on AM2+ CPUs. And yes, improvement is seen in memory intensive apps with a higher HT.
The 'BE' microprocessors may be simply overclocked at stock volts by raising the cpu multi - anywhere from 400-600MHz.