Granted the real question becomes storing data not handling that number of requests, but a database that knows where a bunch of dumb files scales really well. (If you look into things this is Facebook's basic approach.)
I'd venture the guess that you'd be talking quite a different budget than a bunch of pizzaboxes in a horizontal setup though. The SAN to handle 5k IOPS alone will set you back by an interesting amount (even more so when you consider mirroring, which you'd probably want to have at that scale). I'd also be worried about the network - GBit/s is probably not going to cut it at that rate anymore.
So, all in all this is precisely why I asked about horizontal scalability.
A setup of 5 machines that handle 1000 reqs/sec each is usually cheaper than a single machine to handle all of the 5000/sec.
PS: Upgrading 10gb Ethernet is not really that expensive now days if he is only linking a few web servers to two databases.
EDIT: To give you some idea what flash can do http://advancedstorage.micronblogs.com/2008/11/iops-like-you... (Granted, it's a stupid video, but 150,000 Read IO's and 80,000 write IO's and 800MB/second of bandwidth on two PCie Cards in 09 / 10 with fusion IO doing the same type of thing today).
The SAN comes into play when a single box can't deliver the IOPS anymore - remember it's not just a matter of adding SSDs. At those rates you start touching the controller and bus limits. Likewise a saturated 10Gb ethernet link causes a significant interrupt-rate (older cards would bottleneck on a single core) that often exposes interesting corner-cases in your OS and hardware of choice.
I'm not saying it's not doable and I know what SSDs are capable of (we just fitted a server with X25's). I'm just saying that your estimate of $10.000 is very optimistic, add a zero and you'll be closer to home. That's because I still think you'd definately be talking an xfire 4600 class machine and a SAN.
Anyways, this is all speculation. Wheels made some reasonable statements that they have it on their radar and I'm definately looking forward to some real-world benchmarks with a concurrent write-load.