You should read the 2-29-00 issue of WS if you can, which has a glowing article on '97 vintage Ports. Included is a special story on Quinta do Noval, which I'm sure contributed to the cause of its high price.
Some of the ratings from that article:
Quinta do Nocal Nacional: 99
Niepoort: 98
Quinta do Noval: 97
Delaforce; Dow: 95
Taylor; Warre: 94
...
Fonseca: 91
Quinta do Noval/Silval: 91
Nacional is in a price category by itself mainly because of the rarity. They will normally make 200-250 cases/vintage, as opposed to 1,000-1,500 of the regular Noval Vintage, & up to several thousand cases by other Port houses.
Nacional is made only from grapes from a small plot of ungrafted vines, descended from survivors of the phylloxera infestation of the 1870s. Aside from that, it is often the best Port of the vintage. Not always, though, & some vintages are not even very good ('78, '80), in which case the high price is only because of its rarity & legendary status, & obviously not "worth" it.
Quinta do Silval is one of the several vineyards included in the Quinta do Noval property. Historically, its grapes have been used in the regular Noval Vintage. Apparently in '95 they started bottling its own Single Quinta Vintage. I don't have '95 ratings handy, but I bought both the regular Noval and Silval that year.
Generally, such a Single Quinta would not have the same complexity as a blended Vintage Port, and would be expected to mature more quickly.
I guess one might call Noval a hybrid, since it is technically a Single Quinta, but has others included in its property...
If you have money to burn, by all means pay $200 for the '97 Noval or $1,000 for the Nacional. I don't & won't. I avoided buying may '97s because: 1. Prices were too high, about 50-100% higher than '94 release prices; 2. I bought a lot of the best '94s, which are generally a bit better; 3. My financial/employment situation was not as good at that time.
I saw only a couple bottles of '97 Noval for sale in Seattle area, at release, for about $150. That is ridiculous compared to the others. If you have that kind of money, it would be better spent on some of these at
www.tinamou.com, for example:
'77 Fonseca, WS 100, $199
'94 Fonseca, WS 100, $169
'94 Taylor, WS 100, $159
'97 Niepoort, WS 97, $79
The '97 Silval should be around $50 or less, & a pretty good wine, too.
I recently bought some '97 Fereirra at an outstanding special price of $110 for 6 bottles.
'85 Noval (WS 86) was the first Vintage Port that I bought, about 9 years ago, on sale for $17.99.
I have tried it many years ago, loved it.
I had '63 Noval several years ago (WS 84, $90), & it was mediocre. That is a good example of a vintage where the regular Noval can differ a lot from the Nacional (WS 100).
The only other Noval that I've had was '70, WS 89, "...Bottles vary from outstanding to disappointing...." This was one of the disappointing ones, but I have a couple more.
I have one each of Nacional '82 (WS 86, $175), '83 (WS not rated, not officially declared, $196) & '85 (WS 95, $200). All are saved for special occasions, & probably the only times I'll spend that much on a bottle of wine.
Most of what I know(?) about Vintage Port has been from reading various sources...buying, drinking & discussing here.
Welcome & good luck. You might also do a search here for some past discussions, including one not too long ago about Single Quintas, LBV, etc.