if you add one later, you might have a problem with cloning your OS from your hdd to the ssd, the recommended procedure is to do a clean install of OS/drivers on the ssd.
If you get a ssd, which is around $100 for 120g, your total is around $1450. But as you said, you need to buy games...so I guess one way to do this is to downgrade the gtx 670
to the MSI gtx 660 ti OC Power Edition, which saves you $100, and you can use that to buy the ssd. You can also choose the HD 7950, but if you play games like BF3 that favor Nvidia, then an the 660 ti will outperform the 7950 a bit.
If you absolutely love the gtx670, then I'd say wait.