Furthermore, don't even listen to the suggestion about an SSD. If you still have an IDE cable in your motherboard there's no way in hell you're on SATA III. You're probably still on SATA I and there's no way you're going to be getting your money's worth out of an SSD. Your entire computer would bottleneck it, including the SATA port. Not to mention at a $200-400 cost you could probably just get an entirely new PC.
SSD's random I/O can't saturate SATAI bandwidth yet, well on the mainstream SSDs at least.
SATA revision 1.0 - 1.5 Gbit/s - 150 MB/s
edit: speaking of interface hard-caps, theres already a soft-cap within the system, even if you did put a faster HDD or SSD, the system is still soft-capped by the speed limit of data transfers from other sources.
HDD to SSD would be bottlenecked by the HDD(180MB/s peak for even the fastest 7200rpm HDDs), network to SSD would be bottlenecked by the network(1gigabit or 128MB/s).
the same thing applies to the latest hardwares, the only way to mitigate the soft-cap is to have the bottlenecks removed.
at the moment the only way for the SSDs to work at their maximum throughput is via SSD-to-SSD transfers and RAM-to-SSD transfers.
but anyway, if this is the case then the only way to boost system performance is to use more ram and go with superfetch, check if you could squeeze in more rams.
superfetch caches recent accessed files, and evicts the least used cached file.
the cache itself is low-priority so if ever the ram is flagged as "needed" then the cache will evict a portion of itself to make room.
the problem with superfetch is though it doesn't keep them after reboot, so it needs to rebuild it every time you do so.
of course, a full upgrade should be more cost efficient than just partial upgrades.