As a general advice, depending on what CPU and how many RAM you own, I highly suggest you to upgrade your GFX card, as for example on this list
German overview of ALL graphics cards ranking so far! your 8400 GS is listed on position 210 (low-end card, is from 2007, only has 512 MB GDDR-3 and that only on a tiny 64-bit connection!) and the current PR 1.0 version demands a WHOOOOLE lot of GPU (GFX) and CPU horse-power (I HIGHLY recommend enjoying PR on Full HD/1920x1080 resolution and high Anti-aliasing settings), so you should have a gooood dual core CPU (at least) and get at least for example an Nvidia GeForce GTS 450 (placed pos. 81, year 2010, has "already" 1024 MB GDDR-
5 on a 128-bit memory interface ! !
But your best choice would be a GTX 650 atm. for around 100 € (don't know your money in Sweden), and if you can find a cheap "GTX 650 Ti" or even "GTX 650 Ti Boost" version for not too much money (100 - 150 € ) it would be awesome!!
I recommend a MSI (brandname), model: "N650Ti TF 2GD5/OC BE (2048MB)" , running @ 1033 MHz,
or the ZOTAC "Geforce GTX 650 Ti Boost 2048MB GDDR5", which is "just" running @ 993 MHz, so a liiiiiittle bit slower than the MSI one, running at 1033 MHz core clock.
Just as information for the EXACT two international product codes:
MSI one: EAN/UPC: 4719072295967, Manufacturer article number: V287-037R
Zotac model: EAN/UPC: 4895173601767, Manufacturer article number: ZT-61201-10M
both are around 125 - 130/140 € here in Germany at least.
and you should have 4 GB (DDR3, best 1600 MHz) RAM installed for no lags.
Cheers!