When I build a system I spend the money on the motherboard. I stay away from 'onboard' stuff, though I don't know anything about built in NIC's.
So when looking at the mobo, pay attention to what CPU's it will take. It may be best to research this on the internet and not on the mobo 'box'. BIOS's often increase the range at which a mobo can operate extending the life of the board.
I like ASUS boards myself, and have no problems with the A7V series.
You really should let the RAM go.
DDRam is the way to go. Plan on purchasing 256mb to start.
More than likely the case you are using now is ATX compatible, pay attention to what Power Supply it has. You really should have a 300w or better power supply these days. If you don't have 300w now, you can just replace the PS in your old case.
So after all this, you may only need to buy a CPU, RAM and a mobo, which you should have NO problems doing for the amount of money you mention.
If you want to buy a HDD, 7200 ATA100 will give the best performance these days.
SKurj