Get the SATA II HDDs, they easier to find than IDE. I would get 7200 rpm HDDs, just because they are cheaper than the 10000 rpm HDDs. You may also want to look at SSDs, but they are really expensive compared to the 7200 rpm HDDs.
Size depends on what you want to do, if you're gaming and general home use, you could get by with 320GB. If you doing videos, DVDs, large files, etc. go for 500GB to 1TB
1TB is the size to go when your looking for performance in a gaming hdd. The drive density helps improve performance by limiting drive head movement. The velociraptors (the only 10k rpm consumer drive i now of) i would consider obsolete because they're in the same price range as good SSDs.
Those MIGHT be a good deal - the price is right for the sizes - but it depends on how old they are and whether they have known problem areas. HDD's tend to last maybe 5 years (plus or minus a lot). So if these are already 3-4 years old, you can't count on using them for a lot longer.
The Caviar Blue and Green lines are very good units, but NOT the fastest WD makes. For that you go to the Black line. Now, on the cache side, there is a wrinkle to consider. There's at least one WD Green unit out with 64 MB of cache, the largest they make. I'm not sure whether WD also had this cache on any Black units. In General, larger cache is good for performance. In fact, that's why they did it on one Green unit - the cache is there to offset the slightly slower performance issues in other parts of that drive. BUT a large cache is most useful for large files and sequential file access so that big chunks of data can move from disk surface to cache RAM in one disk rotation. Your desire, though is speed to load gaming files like maps, etc. Often these are NOT large individual files - they are MANY smaller files that could be anywhere on the disk. So disk parameters like average head seek time and rotational speed (these two combine to yield the average Latency) are more important for your needs than very large cache size. For example, PsyKhiqZero has pointed out that 1 TB dives have an advantage here. Their internal construction is such that all data are on one disk platter, and so a LOT of data is located in one full track that can be copied from disk to cache in a single revolution. If the cache is sized right (to hold the track) and the data layout happens to be optimized (with minimal fragmentation), there is a really good possibility that the next data you want is already in the cache and not somewhere else on the disk. This eliminates some head seeking time, speeding up the overall average performance.
Ultimate speed for gamers, by common legend, involves using VERY high rotational speed drives (10,000 rpm or more) in RAID0 arrays. These systems cost a LOT more than "normal" drives and RAID0 carries an increased risk of TOTAL data loss. Unless you need and can afford the ultimate, I suggest you search out on the web (including here at Tom's) reviews that report actual performance tests and timings on various drives, and on various drive arrangements. Look especially at the tests that relate to your prime need - fast gaming - like Latency, Access Time, Random Read Throughput, etc. Pay special attention to recent data - the only thing pertinent to purchase of new drives, anyway - because drive technology has improved a lot, and recent "regular" drives nearly match the performance of the "real fast" ones of less than 10 years ago. Then decide on the best balance for you of performance and cost.