How flexible? If you can stretch a little you could get dedicated graphics now.
As onboard graphics go, the 790G is way above anything else as far as I'm aware, but it's still miles behind a HD 4670, 9800GT or HD 4830.
Broadly what I'd try to stretch to for around this budget:
A few notes:
Does GP4 work on Vista? Does it work on Windows 64-bit? I've not tried GP4 it for years now.
If it works on Vista, you might want to save the money on the Vista license for now and download the beta of Windows 7 (which is easily stable enough for temporary use for most, and stable enough for many in long-term use (until the beta expires in August). It's a tight budget, so saving £90 for now (or £70 if you go XP Home) releases a lot of funds to save or spend elsewhere. If GP4 works on Vista it should work on Windows 7 too (which when it comes down to it is just a more refined version of Vista - though a very nicely refined one).
2gb of RAM is a bit stingy for 64-bit Windows. The emulation layer which runs 32-bit applications requires more memory than if you were simply running the same on 32-bit Windows. 4gb would be ideal if you do want to go 64-bit. Again, if you choose to run Windows 7 beta for now you could even dual-boot between 32-bit and 64-bit, and choose which works best. I've left your choice in my budget, but I'd be inclined to either spend the extra £10 on a 4gb kit (2x2gb Corsair) or drop down to a 32-bit version of Windows if you're not going to double the RAM at some point. The £10 for double the RAM is attractive, but again depends on how flexible your budget is.
Case: I'd have liked to have gone for the Silverstone TJ08 and a MicroATX motherboard, but that pushes the budget up further. The Zen
is a popular choice, so it must be for a good reason.
Motherboard: If you can avoid it, don't spend more on a board with decent built-in graphics and then spend a further £100 on dedicated graphics at a later point. If you've got the money at the moment, you'll save yourself £20-40 further down the line.
HDD: You won't find any recommendations for Hitachi Deathstars. Avoid Maxtor like the plague too and I'm not a fan of Seagate currently (lots of dying drives and then compounded with firmware cockups which were supposed to fix some issues, but stopped some drives from working at all!). Samsung and Western Digital are currently the only decent choices IMO. Samsung are a bit cheaper usually, but I've had a couple of odd Samsung drives (data is all fine, but sometimes they aren't picked up by the HDD controller), so I stick with WD personally. Both are generally fine though.
Graphics: If going dedicated, you've got real 3 choices. There's the HD 4670 around the £65-70 price point (excellent value), then a nice step in performance if you spend £90-105 on a 9800GT or £95-110 on a HD 4830 (both still very good value for what they give). Again, personally I'm an ATI guy generally, but go with what side you sit on. Sometimes it is better the devil you know, but I couldn't say if either side is better at GP4, or they're both fine.
I'm sure we could get down to £500 with decent dedicated graphics easily enough, but I feel it would be worth spending a little extra here and there and getting a better rounded system.