do you want to save some money and overclock the cpu? or do you want to run fast at stock and spend more?
your 5000BE will overclock to the speed of the 6400, probably a little more with a good mobo.
i'd keep it and use the 5-6 bills to buy a new mobo and card (plus the other stuff you need).
i've heard pretty good things about this mobo:
Newegg.com - Foxconn A7DA-S AM2+/AM2 AMD 790GX HDMI ATX AMD Motherboard - AMD Motherboards 125.00
plus it has phenom support if AMD can get their **** together.
and i'd drop a 4850 or a 4870 in it.
Newegg.com - MSI R4850-512M Radeon HD 4850 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Desktop Graphics / Video Cards 189.00
Newegg.com - SAPPHIRE 100257L Radeon HD 4870 512MB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Desktop Graphics / Video Cards 229.00
the MSI is the only 4850 i would get, it has spectacular cooling.
ram, get some decent ddr2 800/1066 and pick out a hdd and case. which you should have no problem getting if you go with the mobo and 4850, thats only a little over 300 bucks.