first of all,1 GB of RAM would be appropriate for u. Invest in two 512MB modules from either Corsair or TwinMos.
and regarding processor and motherboard, i think,2.8 HT is pretty good in processing power.Here you just need to upgrade your GPU and SMPS. Go for 6600GT with 256MB and a coolermaster 450W SMPS.
And if you still save some bucks,then look out for decent speakers....like Altec Lansing ATP3 or Philips MMS430!
Sell only,when there is the right time for it.I dont think 2.8 with HT is so bad. And by the way....is this a Prescott??
If it is then it would be good to stay with it.