AdioKIP :
My opinion would be the exact opposite of everyone elses. I wouldnt build a machine today without an SSD as the boot drive. I've been building computers for 20 years and SSD's are probably the second best upgrade I've ever seen. While it wont help you out as far as gaming, the overall difference in how your system boots and runs with an SSD compared to a normal hard drive is HIGHLY noticeable. The fact that you're choosing between two cards that are so similiar also makes the choice easier. Get a descent 7950 and it should easily overclock past a stock 7970ghz edition. Both my Sapphire 7950's (1xOC edition and 1xBoost Edition) easily overclock if I really need extra power. Overall the 7970 will give you slightly better performance if you're not overclocking, but the difference gained in gaming from a 7970 vs a 7950 isnt as noticeable as the difference an SSD makes in overall system performance. Also since you're just building, its easier to get the SSD now so you dont have to reinstall your operating system later on.
I agree that the SSD is one of the best upgrades that you can get for your computer, but I have never had a problem installing an operating system on an SSD after building my rig.
I just started building a few years ago and my first rig didnt have an SSD. I bought one, changed the boot order in the BIOS, installed windows, and was done. I left windows on my HDD because if the SSD failed (like they were prone to do when they first became mainstream) I wouldnt have a dead computer. Then I bought a faster SSD using SATA 3, did the same thing, reformatted my original SSD and now use that as my video recording/encoding drive.
Anyway, the point is, I do not think it is hard to add a SSD after building, and sometimes it is even good, so you can set up a redundant OS in the off chance that your drive does die.