probably around $200 i alread have a case, disc drive, HD, PSU
$200...; the previous combo with the X2 3600 was around $85 but don't forget you ned at least $50 of RAM (1G), a decent PCI-e video card and of course your aging PSU would not be safe for you new system in the long run and your old HDD would bottleneck your system pretty often.
In conclusion, I'd either advice you to 1) get the $77, S754 combo and raise a better budget for a total upgrade later (this gives you a better CPU for most games than the X2 3600+, spares you money now and allows for a better and cheaper upgrade later)
2)Add some money and buy all that stuff, but still keeping in mind to upgrade PSU and HDD later (a more future-proof system but it's more expensive + more urgent to change other hardware)
Don't have to add that I'd go for the socket 754 system if I was a gamer with only $200