At Softlayer 16GB is $350/mo more than 2GB, _but_ they almost always have a special double/double for the same price (double the HDDs and double the RAM). So I just bought a 16GB server for userscripts.org for the 8GB price ($150/mo).
Why not just buy a machine and just colo it Softlayer? That way when you do need an additional 8GB, you can pay whatever the current price of 8GB is. These days you can build a 4 to 8 core machine with 16GB ecc memory for around $1500. Hardware is cheap these days... you should be putting most of the money into electricity & bandwidth.
For me it is worth it not to have to deal with it.
For $529/mo I got:
2x Intel Xeon-Clovertown 5320L-QuadCore [1.86GHz]
4x Seagate Cheetah ST373455SS (10K SCSI drives)
Hardware RAID controller
16GB RAM
2TB transfer per month.
100Mbps uplink
Peace of mind
Sure I could get everything but that last item for less. There is an inflection point where it is more economical to run my own hardware again but I'm not there. For a few hundred more than cost when issues occur a competent support person is helping me within minutes.
When (not if) there are issues with my servers, driving to a colo would eat my savings. When you have a hardware issue at softlayer they fix it. They don't have to wait for new hardware as they have a stock.
I'd rather be spending my time coding and working on features. Having your own non-mission colo-ed box is awesome, but the cases where dedicated leasing or AWS doesn't make sense are fewer and fewer for bootstrapers.
Note: userscripts.org is 3 years old. It ran on a colo-ed box for a year. Then a $200 serverbeach box. But it just outgrew that ( http://userscripts.org/articles/22-2008-overview for 2008 review with stats )
If you really want something that is peace-of-mind, you should just use something like rightscale.com. We have a giant ad server on there load balanced across 40 ec2 instances running perl code, seems to work well so far. Sometimes an instance will get fussy and we'll just create a new one from scratch by running scripts that essentially fetch everything needed from CPAN and installs, configures, and boots in one step. It's definitely a more expensive solution, but if you want peace of mind, that's probably the way to go.
About a year ago I spent many hours talking with a rep who came back with the same boxes and prices we could have configured with their online system.
Last week I spent a couple hours chatting with a rep for userscripts.org's new box and he did better than the online prices.
I think the difference last year I approached with general requirements, so they spent all their time speccing out multiple systems and figuring out what was the best fit. My second experience I knew what I wanted, so I was able to ask for deals based on specific hardware choices.
http://www.softlayer.com/specials.html