Upgrading A Socket 604 CPU

[Zaune]

Cross-posting from Reddit, and the Arch-Rival. (Original post.)

So, I've got an ex-lease workstation (an HP XW8200 to be exact) sporting a brace of Intel Xeons at 3.60GHz, which sounded a lot more impressive than it actually turned out to be once I found some benchmark information.
Unfortunately, I haven't been able to find out much else useful; Passmark doesn't have a category for Socket 604 anymore and Intel's website isn't much help either. I therefore have the following questions:

1. What's the best Socket 604 CPU available, and how much should I expect to pay for a pair of them?
2. How much of an actual performance improvement am I likely to get?
3. Am I going to need a PSU better than 600w?
4. Is this actually worth the hassle, or should I just bite the bullet and start saving up for a new PC?

Extra information in response to a question asked in the comments: I've got a Geforce GTX480 graphics card (on order, current GPU is an HD5450), 8GB of RAM, a single 1TB hard drive (no SSD yet) and a Blu-Ray drive that I almost never actually use. Cooling is all stock, with three case fans.

[Starglider]

That was the last-gen pin grid array socket, before everyone moved to land grid array. The good news is that the 3.6 GHz Northwoods were the fastest Pentium 4s ever made for single-threaded benchmarks (excepting a few cache-specific ones). The bad news is that the dual-core Pentium 4s didn't arrive until the following (Prescott) generation. P4 had horrible IPC and the dual-socket versions were still saddled with a shared FSB, also it only supported the initial sucky version of DDR2 that was actually slower than decent DDR1 memory. CPU & memory performance will be comparable to a contemporary (cheap dual-core ~2 GHz) netbook. This is why I only bought Opterons servers at that time, they were faster to start with and AMD's better socket stability meant they were forward compatible with the dual-core Opterons (did that upgrade myself on several machines).

An SSD is the only meaningful upgrade. A GTX 480 is already going to be held back by the CPU and first-gen PCIe + FSB performance, anything newer would definitely be a waste.

[Zaune]

I've had a few other people elsewhere suggest later Xeons like the 7150N X7460; will those actually work, even somewhat hobbled by the low FSB? I've seen used ones for surprisingly little on eBay.

[Starglider]

I've had a few other people elsewhere suggest later Xeons like the 7150N X7460; will those actually work, even somewhat hobbled by the low FSB? I've seen used ones for surprisingly little on eBay.

No idea, I clearly didn't notice at the time or didn't remember that a PGA version of the Pentium D was released. That said I'd be surprised if it did work, because the P4-D was internally two Pentiums on a shared FSB similar to the original dual-socket configuration; dual P4-Ds were equivalent to the old quad-socket shared FSB boards which required a different northbridge. Even if it did work 4 cores on one FSB kills multithreading performance for anything but the smallest working sets. Finally most games still prefer a faster dual-core over a slower quad-core, so might not be relevant to your use cases anyway (video encoding likes more cores but the ISA extensions on a modern CPU blow away those old P4s).

[Zaune]

The most CPU-intensive thing I do right now is playing Dwarf Fortress, so multithreading taking a hit isn't a complete dealbreaker.

[DaveJB]

No idea, I clearly didn't notice at the time or didn't remember that a PGA version of the Pentium D was released. That said I'd be surprised if it did work, because the P4-D was internally two Pentiums on a shared FSB similar to the original dual-socket configuration; dual P4-Ds were equivalent to the old quad-socket shared FSB boards which required a different northbridge. Even if it did work 4 cores on one FSB kills multithreading performance for anything but the smallest working sets. Finally most games still prefer a faster dual-core over a slower quad-core, so might not be relevant to your use cases anyway (video encoding likes more cores but the ISA extensions on a modern CPU blow away those old P4s).

There were some dual-core Socket 604 Xeons produced back in 2005 (the Pentium D was only ever S775), but no-one took any notice of them because they were spectacularly awful in comparison to the dual-core Opterons that were being produced at the time. Heck, the dual/quad-core Opterons even shat all over the Core 2-based Xeons thanks to the latter still being hobbled by the multi-die shared FSB approach; it wasn't until the Nehalem Xeons showed up that Intel retook the lead in that sector.