speaking from personal experience (and not from source citing)... there are no realistic downsides to using raid 0 in a desktop environment... your two biggest concerns are heat generation, and risk of failure... ...if you put 2 or more drives close together, with no form of cooling, youre more likely to experience failure (as they do generate a substantial amount of heat)... ...theres also the issue with onboard raid controllers being more likely to cause data corruption, (though only that ive read from other posters)...
the concern over slower seek times in a raid configuration, is only about 1-2ms slower than compared to a single drive, yet the throughput performance nearly doubles, because raid 0 does scale fairly well, depending on the controller used... in any situation where hdd performance is the limiting factor, (which is typically the case, as its the slowest component in your whole system thats used regularly)... theres really no reason to not go with raid 0...
ideally the drives in raid should be identical (both in speed and capacity)... otherwise your performance wont be optical (being held back by the least capable drive in the array), your additional capacity will be unused on the larger drive too... eg, 40gb and 60gb hdd in raid 0 = 80gb, not 100gb
raid array stripe sizes are of concern too... you would set them according to your typical file size usage to acheive the best performance... lots of large files = large stripe size... lots of small files = small stripe size... windows partition = small to medium stripe size... gaming partition = large stripe size...
i believe the difference between 7200.9 and 7200.10, is that the 7200.10 uses perpendicular recording to improve capacity (and possibly performance maybe, though increased capacity inherantly improves throughput performance anyhow)...
any situation that accesses your hdd, would do well to be on a raid 0 array, even with just 2 drives, compared to a single drive setup (unless the drives themselves are rediculously outdated to begin with, ie, 4200, 5400 rpm... whichcase, yeah)... because even with small files being loaded and accessed, theres still an improvement... even if it is small.