However, this is only about 60 % utilization, it's frustratingly close to being able to double this. (It would need about 10K vs. my 9K LUTs.) The next best Spartan3 has 1200K gates vs this 500K, so it might be able to quadruple the units.
I think you need a Spartan6 to do any serious mining, but even then you should check the number of logic units, the series has some low-end models as well.
That reminds me - have you managed to synthesize your code for a Spartan 6? I tried it, but it bailed out early on with a cryptic message about synthesis failing and no other information I could find. Rumour has it the Spartan 6 support may be more temperamental than for earlier generations. (Not that I have an FPGA to run this on anyway!)