Upgrading CPU, need some advice

Current specs:
Asus A8N-E (socket 939, oh no!)
Athlon 64 3700+
2GB Corsair XMS PC3200
GeForce 7800GTX

I'm thinking about getting in on that dual core goodness by dropping either an X2 or an Opteron in to my current motherboard. Annoyingly the only socket 939 dual core processors available on newegg include:
X2 3800+ Manchester-$58.99
Opteron 180 Denmark-$169.99
Opteron 185 Denmark-$235.00

I'm not really ready to fully upgrade my computer as it appears that a I'd have to buy new RAM along with a motherboard and processor to extract full potential out of something like a core 2 setup. My understanding is that I'll see a significant performance increase by swapping out my 3700+ for a dual core CPU.

I'm having alot of trouble finding benchmarks for these Opterons though. I'm willing to fork out the money for something like the Opteron 180, the 185 seems a little stupid as I doubt 200mhz is worth $65. My question is would it be worth it to fork out the money and go for an Opteron over an X2? Also does anyone know of any retailers with a greater selection of socket 939 CPUs? What processor do you think it's worth buying at this point?

Thanks!

The 60 dollars for the 3800 is probably worth it, tho it's a slightly slower clock speed, if you can get the 4200 for around the same price that's the best option as it's the same speed as your current chip but dual cored. I bought a 4600 to upgrade from a 3700 last year and I dunno if it was worth it really, I think you're better off saving and getting a new motherboard etc. Definitely don't spend any more than 100 dollars on the chip in any case.