In two years, you will have to change from XP, anyway. I'm not fond of the 7 start menu, but the OS is generally a step forward. Check out Classic Shell. IMO, the up button and file dialog changes are the best part, but they also have a rebuilt old-style start menu, with less clicking to get places. You can turn UAC off, but you'll be better off to get used to it. If Apple and Ubuntu users can handle an extra dialog here and there, so can you . Also, setting up an SSD for Windows 7 is as easy as installing the OS from the disc.
I would make a point, if sticking with XP for now, to align the partitions properly. Performance should not change much, but I would be concerned about XP causing higher wear compared to aligned partitions, since XP is a fairly write-happy OS. It will bottleneck the SSD some, but OTOH, an HDD having to physically move the head around is still the biggest bottleneck to worry about.
If you want 500MB/s reads, you'll need either RAID, a PCIE-e SSD, or a nice controller card. Most cards are not going to perform as well as an Intel or AMD IO hub. For the cost of one that I would trust to perform well enough (LSI 4-port 6Gbps would be my minimum), you'd have to spend half the cost of a CPU+mobo+RAM upgrade.
IMO, if you are not already using SSDs, get an Intel or Samsung (being limited to 3Gbps, a 320 for the right price would be a good bet; I'd get a Samsung 830 if considering a CPU+mobo upgrade in the near future, assuming brand and model price differences mimic what we see in the U.S.), and deal with the torture of knowing that a more expensive drive in a brand new computer might be faster, while you try to figure out what to do while you aren't waiting on arms to find data tracks on a spinning platter. Seriously, you're dealing well in excess of an order of magnitude of improvement in all-around storage performance, in some cases in excess of two orders of magnitude. Large file copies are the only situation where a good 3Gbps SSD will only be 2-4 times typical HDD speeds.
Technically, 3Gbps will bottleneck both random and sequential loads, but well, context matters. On a power user's desktop, the differences between SSD controllers and firmware will present themselves much more than 3Gbps v. 6Gbps, except in large copy jobs. 6Gbps would be nice and all, but it's not worth losing sleep over, unless 200+MB/s file transfers will cost you money (in which case, you could afford a mobo+CPU+RAM upgrade, since it would pay for itself over some time period).