I always prefer to order all my parts within a few days of each other. You should never buy them piece meal, as in general, they lose value with time. Effectively, the moment the parts hit my door mat, they are start losing value.
Also bear in mind that of all components, SSDs are falling fastest, in price. I got my Cruical 128GB (ordered 2 days ago), for £90. Had I bought it 2 months ago, I probably would've had to pay 25% more.
Apparently, what is happening is that there is a price war going on with SSDs (much like there was with HDDs, a year or so ago). So, expect prices to plummet fast. In fact, I expect SSDs to begin to take a big chunk out of the HDD market, as the HDD prices do not appear to be heading back to their pre-flood prices.