Most graphics cards (perhaps ALL ATi and NVIDIA models) come with DVI-to-VGA adapters. There is no such thing as an HD monitor. All "HD" means (in terms of displays) is that the screen has a resolution of 1280x720 or 1920x1080. Most computer monitors have resolutions of much higher than that, but all it means is that the monitor can display that many horizontal lines and that many vertical lines, or more. So really "HD monitor" doesn't mean anything.
If you are referring to an old CRT (big heavy tube) monitor (instead of a modern, flat, light LCD monitor), then the answer is yes. Graphics cards all do the same thing, and they have been doing the same thing for years, except now, they are more than just display outputs, they are gaming engines.
All DirectX 10 hardware supports 9, 8, and 7. Everything is backwards compatible. All "DX10 support" means is that gaming can be accelerated using the new DirectX 10 API, which is exclusive to Vista. Basically, it just looks somewhat nicer than the same game rendered in DirectX 9, but there is a good chance you would not know the difference, plus very few games support this.
If you are not a gamer,
none of this is relevant to you. If you would like a card that will just suit your everyday needs, but you are planning on keeping it for years to come, this is the card for you. Note: this is not a gaming-oriented card.
NVIDIA GeForce 7300GT
This card is enough to enhance and optimize playback of video (especially HD video) on your computer, which is perfect for those whose multimedia purposes on their PC's are strictly for audio and video.
If you are a gamer,
(which I assume you are NOT, because you are still running a Radeon 9800Pro on an AGP chipset), then your choices are limited. I would imagine you haven't gamed in a while and would like to start again, in which case: First off, what processor are you running? If it is a Celeron, forget about gaming. If it is a Pentium 4 (2.8GHz or higher)
or an AMD Athlon 64 equivalent, and
you have at least 1.5GB of RAM (for keeping up with modern gaming and even just to keep up with new and future operating systems like Windows Vista, you will find it quite necessary to upgrade to at least 2GB)
then your best option is this:
the ATi Radeon HD3850 AGP
This card will run the most recent games quite smoothly, and should run even high-end, graphics intensive games quite decently. Keep in mind that if you are running a single core processor such as a Pentium 4 or Athlon 64 or Sempron, the performance will be quite bottlenecked. However, you should still experience a VERY
dramatic boost in performance. To compensate for the bottleneck, you should expand your RAM memory to the highest possible.
In all honesty though, if you want to start gaming again, or you are a gamer, forget this system all together, because upgrading all the parts (AGP cards have been phased out and are becoming very rare, and old RAM memory could get expensive) will cost more than simply investing in a new system which would be MANY TIMES
more powerful than an old PC running with an AGP interface.
In otherwords, $300 in upgrading an old PC with AGP that previously ran a 9800Pro would not be even remotely close to as powerful as buying a new $300 dual-core PC. And thus, it's not a wise investment.
Back to what I said earlier, if you are NOT
a gamer, then invest in the card i found above if you want not only a display adapter, but also some longevity for your machine. (that card will run Vista Aero very well, and enhance playback of video which your PC may not be able to keep up with otherwise) It is also slightly more powerful than your 9800Pro, as a bonus.
Good luck, Poobear!
PS: to the other users of CF who are wondering why i'm suggesting a 7300GT and not something less powerful is because I cannot find the AGP version of the 8400GS or anything similar. I'm not suggesting a 5-series or 6-series card because I want Poobear's investment to help keep his PC usable longer, and the 5 and 6-series cards are ridiculously outdated now.