First off, you're kind of making a duplicate post about the same topic:
http://www.tomshardware.com/answers/id-3526352/asus-m2n68-se2-pegatron-ap480-motherboard.html
But that's up to the moderators to detect.
You're about at the limit for that Asus motherboard:
https://www.asus.com/us/Motherboards/M2N68AM/specifications/
-- RAM is maxed out (4GB maximum), & even switching to DDR2-1066 from your current DDR2-800 will have little to no effect.
-- You already have an SSD, but you're limited anyway because your motherboard only supports SATA II (3Gbps) connections. I would definitely add that HDD in as additional storage temporarily, but you could always add another drive for some extra space.
-- CPU-wise, there's very little you can do. Although technically a Socket AM2/AM2+ motherboard, Asus does say it'll support some of the Socket AM3 CPUs (https://www.asus.com/us/Motherboards/M2N68AM/HelpDesk_CPU/). However, you already have the fastest Phenom II X4 possible with that motherboard, & the only X6 option that doesn't cut your core speeds down is the Phenom II X6 1090T. However, that only moves you from a 4th-tier CPU to a 3rd-tier CPU; a single-tier upgrade is usually frowned upon, because you don't get a whole lot of improvement on it. And that's even assuming you can find one to purchase.
That Pegatron motherboard might be a better bet (http://www.hyrican.de/wordpress/wp-content/uploads/Downloads/Handb%C3%BCcher/Mainboard-Handbuch/Pegatron/Pegatron%20AP480-S/AP480-S_20080718.pdf). Although I can't determine for sure what the CPU support would be, it
probably supports your current CPU. There are 2 main reasons, however, to consider using it instead:
-- more SATA ports. Although you're still limited to SATA II speeds, you have 6 ports instead of 4.
-- more RAM. It's still limited to DDR2 RAM, but the maximum it can use is 8GB. That won't matter if you have 32-bit Windows installed, but having 8GB available means you have the option to go to a 64-bit version of Windows.
In any case, that's a very old system. Whatever money you spend trying to upgrade it might be better spent on a new system like a Ryzen 3 or so.