Move SQL to its own box.
With 25-30 million transactions per day you must be averaging something like 350 database transactions per second, which is pretty considerable.
It depends on what you are doing with the database, but unless your queries are pulling large datasets, you are probably more disk limited than you are processor limited.
You'd probably see more of a performance boost with a better disk infrastructure than you would with 4 processors, and you'd also avoid having to get a SQL Enterprise license, which would cost more than the hardware.
High end SQL setups should typically have at least 3 distinct physical volumes -- OS, transaction log, and data. Depending on the data, it may be further segmented to more physical volumes. In our environment, we typically have the OS on a mirror, the log files on a RAID1 or 10, and the data on a RAID5. The more drives the merrier as the more drives the better your performance. If budget allows, go for RAID10 on both transaction log and data arrays.
I wouldn't suggest putting the transaction logs on IDE Drives as they will become a bottleneck -- the log needs to be written before the data is written. Initially you may get a performance boost just because you'll be eliminating some contention all around, but it isn't a good long term strategy.
At the levels your talking about you need many more smaller drives, not fewer larger drives.
From a memory perspective it also comes back to query patterns -- large querysets will benefit from extra ram as SQL will use it for more data caching. The ram won't help your write performance.
You should also be moving to a platform that you can readily expand -- any solid raid platform should let you expand the RAID10 and RAID5 arrays 'live', which lets you add drives as you need them for performance or capacity reasons.
Don't forget about future expansion as well -- clustering is a logical requirement as your site grows and the downtime becomes expensive (or more expensive than it is already, as the case may be) -- make sure your infrastructure will support it.