I realize that this topic has been covered quite a bit in the past, but I have a couple of questions still after reading through some of the other threads.

First, I know that the oil seals between my 2009 GS500F and a, say 1989 GS500E are the same part.  How about the dust seals?  I checked out the parts fiche and it says they are both 51173-01D00. However, the 1989 model says that part was replaced with part 51173-08C20.  Huh? How does the 2009 model use an older part than the 1989 model?

Also, I'm going to be buying some stiffer Sonic Springs.  Do they come in a pair or do I have to buy one for each fork leg?

As an aside, I called my bike shop to get a price on the seals. They quoted me $45.88 for a set of oil seals and $40 EACH for the dust seals!!  Weird, because their website parts fiche has completely different prices.
http://www.apachemotorcycles.com/fiche_section_detail.asp?section=1044589&category=Motorcycles&make=SUZUKI&year=2009&fveh=28479

Maybe they just haven't updated their website in awhile?

Springs come as a pair.

Whichever seals and caps you get they'll work. Don't worry about parts superseded. Whichever is available is OK if you're going OEM.

Pyramind brand fork seals and dust caps are $30 or less for a set. They're good. Weird your dealer charges more for the dust cap than the fork seal. You can buy just the dust caps for under $20 a pair.

Sounds like the dealer is quotin full retail plus.

Adidas. I've seen some posts online that say Pyramid seals are no good. However, I trust your judgment over that of random people on the interwebs (even though I've never met you, I've read this forum long enough to know to trust your expertise in GS parts). 

So, these are good and a good price?
http://www.pyramid-parts.com/products/fork-seal-dust-seal-combo-kit-010-068

Based on my experience, they are fine.

Maybe others can suggest their favorite fork seal.

Cool. I got the go-ahead from the boss (Mrs. ASUDave) to purchase the parts needed to replace the seals and upgrade the suspension.  And, as a bonus, since the price of the seals was much less than anticipated, she okayed the purchase of a used R6 rear shock from fleabay.   

This is what I bought:
Seals http://www.pyramid-parts.com/products/fork-seal-dust-seal-combo-kit-010-068
Springs http://www.sonicsprings.com/catalog/product_info.php?cPath=31_65&products_id=133&osCsid=66d2e93d812ac06f1123baaa2d4faa94
Fork Oil http://www.sonicsprings.com/catalog/product_info.php?products_id=486&osCsid=66d2e93d812ac06f1123baaa2d4faa94
Rear Shock http://www.ebay.com/itm/280888918784