Asrock makes the n68c-s ($41.99 plus shipping at newegg) that can unlock a core and has 2 ddr2 and 2 ddr3 slots; reviews are pretty good, but unlocking is hit or miss. Similar boards by other brands will have the same chipset, so this asrock is as good as it gets.
Have you tried selling your old ddr2? You can post it free on craigslist or anandtech's for sale forum. You don't need but 2 gb of ddr3 to get started. If you get $35 for your old ram, you can get 2 gb of ddr3 (single stick) for around $50, and add a second stick later. Newegg has a biostar 785 board for only $59.99 with email promo code if you register with them. That price will probably end today, but they will have more deals on black friday weekend. The biostar board has unlocking abilities; I have used their t-force series boards, and they are all pretty good, including the 870. Only negative is that they don't pay their rebates. I'm still waiting on four of them, some are nine months old.